diff --git a/src/ifcviewer-full/CMakeLists.txt b/src/ifcviewer-full/CMakeLists.txt
index ad5a5083ad..f578d96f70 100644
--- a/src/ifcviewer-full/CMakeLists.txt
+++ b/src/ifcviewer-full/CMakeLists.txt
@@ -34,7 +34,3 @@ set_target_properties(IfcViewerFull PROPERTIES
 target_link_libraries(IfcViewerFull PRIVATE IfcViewer)
 
 install(TARGETS IfcViewerFull EXPORT ${IFCOPENSHELL_EXPORT_TARGETS})
-
-if(BUILD_IFCVIEWER_TESTS)
-    add_subdirectory(tests)
-endif()
diff --git a/src/ifcviewer-full/Federation.cpp b/src/ifcviewer-full/Federation.cpp
deleted file mode 100644
index a72f783103..0000000000
--- a/src/ifcviewer-full/Federation.cpp
+++ /dev/null
@@ -1,308 +0,0 @@
-/********************************************************************************
- *                                                                              *
- * This file is part of IfcOpenShell.                                           *
- *                                                                              *
- * IfcOpenShell is free software: you can redistribute it and/or modify         *
- * it under the terms of the Lesser GNU General Public License as published by  *
- * the Free Software Foundation, either version 3.0 of the License, or          *
- * (at your option) any later version.                                          *
- *                                                                              *
- * IfcOpenShell is distributed in the hope that it will be useful,              *
- * but WITHOUT ANY WARRANTY; without even the implied warranty of               *
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                 *
- * Lesser GNU General Public License for more details.                          *
- *                                                                              *
- * You should have received a copy of the Lesser GNU General Public License     *
- * along with this program. If not, see <http://www.gnu.org/licenses/>.         *
- *                                                                              *
- ********************************************************************************/
-
-#include "Federation.h"
-
-#include <QDir>
-#include <QFile>
-#include <QFileInfo>
-#include <QSaveFile>
-#include <QJsonArray>
-#include <QJsonDocument>
-#include <QJsonObject>
-#include <QJsonValue>
-#include <QUuid>
-
-namespace {
-constexpr const char* kSchema = "ifcfed/1";
-
-QString resolvePath(const QString& fed_dir, const QString& stored) {
-    if (stored.isEmpty()) return stored;
-    QFileInfo fi(stored);
-    if (fi.isAbsolute()) return QDir::cleanPath(stored);
-    return QDir::cleanPath(QDir(fed_dir).absoluteFilePath(stored));
-}
-
-// Returns abs_path relative to fed_dir if abs_path lives under fed_dir,
-// otherwise returns abs_path unchanged.
-QString relativizePath(const QString& fed_dir, const QString& abs_path) {
-    QString fed_canon = QDir::cleanPath(fed_dir);
-    QString abs_canon = QDir::cleanPath(abs_path);
-    if (!fed_canon.endsWith('/')) fed_canon += '/';
-    if (abs_canon.startsWith(fed_canon)) {
-        return QDir(fed_canon).relativeFilePath(abs_canon);
-    }
-    return abs_canon;
-}
-}  // namespace
-
-Federation::Federation(QObject* parent) : QObject(parent) {}
-
-QString Federation::generateId() {
-    return QUuid::createUuid().toString(QUuid::WithoutBraces);
-}
-
-bool Federation::isFederationPath(const QString& path) {
-    return path.endsWith(".ifcfed", Qt::CaseInsensitive);
-}
-
-void Federation::clear() {
-    file_path_.clear();
-    name_.clear();
-    created_ = QDateTime();
-    modified_ = QDateTime();
-    models_.clear();
-    has_home_view_ = false;
-    home_view_ = HomeView{};
-    setDirty(false);
-}
-
-void Federation::markClean() {
-    setDirty(false);
-}
-
-void Federation::setDirty(bool d) {
-    if (dirty_ == d) return;
-    dirty_ = d;
-    emit dirtyChanged(d);
-}
-
-const Federation::Model* Federation::findById(const QString& fed_id) const {
-    for (const auto& m : models_) {
-        if (m.id == fed_id) return &m;
-    }
-    return nullptr;
-}
-
-QString Federation::addModel(const QString& source_path,
-                              const QString& display_name) {
-    if (source_path.isEmpty()) return {};
-    if (isFederationPath(source_path)) return {};  // no nested federations
-
-    Model m;
-    m.id = generateId();
-    m.display_name = display_name.isEmpty()
-        ? QFileInfo(source_path).fileName()
-        : display_name;
-    m.source_kind = "local";
-    m.source_path = QDir::cleanPath(QFileInfo(source_path).absoluteFilePath());
-    models_.push_back(std::move(m));
-    setDirty(true);
-    return models_.back().id;
-}
-
-void Federation::removeModel(const QString& fed_id) {
-    for (auto it = models_.begin(); it != models_.end(); ++it) {
-        if (it->id == fed_id) {
-            models_.erase(it);
-            setDirty(true);
-            return;
-        }
-    }
-}
-
-void Federation::setHomeView(const HomeView& hv) {
-    home_view_ = hv;
-    has_home_view_ = true;
-    setDirty(true);
-}
-
-void Federation::clearHomeView() {
-    if (!has_home_view_) return;
-    has_home_view_ = false;
-    home_view_ = HomeView{};
-    setDirty(true);
-}
-
-bool Federation::load(const QString& path,
-                      QStringList* warnings,
-                      QString* err) {
-    QFile f(path);
-    if (!f.open(QIODevice::ReadOnly)) {
-        if (err) *err = QString("Cannot open %1: %2").arg(path, f.errorString());
-        return false;
-    }
-    QByteArray bytes = f.readAll();
-    f.close();
-
-    QJsonParseError pe;
-    QJsonDocument doc = QJsonDocument::fromJson(bytes, &pe);
-    if (doc.isNull() || !doc.isObject()) {
-        if (err) *err = QString("Parse error in %1: %2").arg(path, pe.errorString());
-        return false;
-    }
-    QJsonObject root = doc.object();
-
-    clear();
-    file_path_ = QDir::cleanPath(QFileInfo(path).absoluteFilePath());
-    QString fed_dir = QFileInfo(file_path_).absolutePath();
-
-    QString schema = root.value("schema").toString();
-    if (schema != kSchema && warnings) {
-        *warnings << QString("Unknown schema '%1' (expected '%2'); attempting to load anyway.")
-                         .arg(schema, kSchema);
-    }
-
-    name_ = root.value("name").toString();
-    created_ = QDateTime::fromString(root.value("created").toString(), Qt::ISODate);
-    modified_ = QDateTime::fromString(root.value("modified").toString(), Qt::ISODate);
-
-    QJsonArray arr = root.value("models").toArray();
-    for (int i = 0; i < arr.size(); ++i) {
-        if (!arr[i].isObject()) {
-            if (warnings) *warnings << QString("models[%1] is not an object; skipping.").arg(i);
-            continue;
-        }
-        QJsonObject mo = arr[i].toObject();
-
-        Model m;
-        m.id = mo.value("id").toString();
-        if (m.id.isEmpty()) m.id = generateId();
-        m.display_name = mo.value("display_name").toString();
-
-        QJsonObject so = mo.value("source").toObject();
-        m.source_kind = so.value("kind").toString("local");
-        if (m.source_kind != "local") {
-            if (warnings)
-                *warnings << QString("models[%1]: unsupported source kind '%2'; entry kept but not loaded.")
-                                 .arg(i).arg(m.source_kind);
-            // Keep raw stored path so save() round-trips correctly.
-            m.source_path = so.value("path").toString();
-            models_.push_back(std::move(m));
-            continue;
-        }
-
-        QString stored = so.value("path").toString();
-        if (stored.isEmpty()) {
-            if (warnings) *warnings << QString("models[%1]: missing source.path; skipping.").arg(i);
-            continue;
-        }
-        m.source_path = resolvePath(fed_dir, stored);
-
-        if (m.display_name.isEmpty())
-            m.display_name = QFileInfo(m.source_path).fileName();
-
-        QJsonValue tv = mo.value("transform");
-        if (tv.isArray()) {
-            QJsonArray ta = tv.toArray();
-            if (ta.size() == 16) {
-                for (int k = 0; k < 16; ++k) m.transform[k] = float(ta[k].toDouble());
-                m.has_transform = true;
-            } else if (warnings) {
-                *warnings << QString("models[%1]: transform must be 16 floats; ignored.").arg(i);
-            }
-        }
-
-        QJsonValue vv = mo.value("visible");
-        if (vv.isBool()) m.visible = vv.toBool();
-
-        models_.push_back(std::move(m));
-    }
-
-    QJsonValue hv = root.value("home_view");
-    if (hv.isObject()) {
-        QJsonObject ho = hv.toObject();
-        QJsonArray ta = ho.value("target").toArray();
-        HomeView v;
-        if (ta.size() == 3) {
-            v.target = QVector3D(float(ta[0].toDouble()),
-                                 float(ta[1].toDouble()),
-                                 float(ta[2].toDouble()));
-        }
-        v.distance = float(ho.value("distance").toDouble(50.0));
-        v.yaw      = float(ho.value("yaw").toDouble(45.0));
-        v.pitch    = float(ho.value("pitch").toDouble(30.0));
-        home_view_ = v;
-        has_home_view_ = true;
-    }
-
-    setDirty(false);
-    return true;
-}
-
-bool Federation::save(const QString& path, QString* err) {
-    QString abs_path = QDir::cleanPath(QFileInfo(path).absoluteFilePath());
-    QString fed_dir = QFileInfo(abs_path).absolutePath();
-
-    QJsonObject root;
-    root["schema"] = kSchema;
-    if (!name_.isEmpty()) root["name"] = name_;
-
-    if (!created_.isValid()) created_ = QDateTime::currentDateTimeUtc();
-    modified_ = QDateTime::currentDateTimeUtc();
-    root["created"]  = created_.toUTC().toString(Qt::ISODate);
-    root["modified"] = modified_.toUTC().toString(Qt::ISODate);
-
-    QJsonArray arr;
-    for (const auto& m : models_) {
-        QJsonObject mo;
-        mo["id"] = m.id;
-        mo["display_name"] = m.display_name;
-
-        QJsonObject so;
-        so["kind"] = m.source_kind;
-        if (m.source_kind == "local") {
-            so["path"] = relativizePath(fed_dir, m.source_path);
-        } else {
-            // Round-trip raw value for unsupported kinds.
-            so["path"] = m.source_path;
-        }
-        mo["source"] = so;
-
-        if (m.has_transform) {
-            QJsonArray ta;
-            for (float v : m.transform) ta.append(double(v));
-            mo["transform"] = ta;
-        }
-        if (!m.visible) mo["visible"] = false;
-
-        arr.append(mo);
-    }
-    root["models"] = arr;
-
-    if (has_home_view_) {
-        QJsonObject ho;
-        QJsonArray ta;
-        ta.append(double(home_view_.target.x()));
-        ta.append(double(home_view_.target.y()));
-        ta.append(double(home_view_.target.z()));
-        ho["target"]   = ta;
-        ho["distance"] = double(home_view_.distance);
-        ho["yaw"]      = double(home_view_.yaw);
-        ho["pitch"]    = double(home_view_.pitch);
-        root["home_view"] = ho;
-    } else {
-        root["home_view"] = QJsonValue();  // null
-    }
-
-    QSaveFile f(abs_path);
-    if (!f.open(QIODevice::WriteOnly | QIODevice::Truncate)) {
-        if (err) *err = QString("Cannot write %1: %2").arg(abs_path, f.errorString());
-        return false;
-    }
-    f.write(QJsonDocument(root).toJson(QJsonDocument::Indented));
-    if (!f.commit()) {
-        if (err) *err = QString("Failed to commit %1: %2").arg(abs_path, f.errorString());
-        return false;
-    }
-
-    file_path_ = abs_path;
-    setDirty(false);
-    return true;
-}
diff --git a/src/ifcviewer-full/Federation.h b/src/ifcviewer-full/Federation.h
deleted file mode 100644
index 21bb6ba3bd..0000000000
--- a/src/ifcviewer-full/Federation.h
+++ /dev/null
@@ -1,105 +0,0 @@
-/********************************************************************************
- *                                                                              *
- * This file is part of IfcOpenShell.                                           *
- *                                                                              *
- * IfcOpenShell is free software: you can redistribute it and/or modify         *
- * it under the terms of the Lesser GNU General Public License as published by  *
- * the Free Software Foundation, either version 3.0 of the License, or          *
- * (at your option) any later version.                                          *
- *                                                                              *
- * IfcOpenShell is distributed in the hope that it will be useful,              *
- * but WITHOUT ANY WARRANTY; without even the implied warranty of               *
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                 *
- * Lesser GNU General Public License for more details.                          *
- *                                                                              *
- * You should have received a copy of the Lesser GNU General Public License     *
- * along with this program. If not, see <http://www.gnu.org/licenses/>.         *
- *                                                                              *
- ********************************************************************************/
-
-#ifndef FEDERATION_H
-#define FEDERATION_H
-
-#include <QObject>
-#include <QString>
-#include <QStringList>
-#include <QDateTime>
-#include <QVector3D>
-
-#include <array>
-#include <vector>
-
-// In-memory representation of an .ifcfed file (IFC federation).
-//
-// A federation is a named, ordered list of model sources plus an optional
-// "home view" camera state.  Source paths can be relative (resolved against
-// the .ifcfed's directory) or absolute.  Save() reserialises paths relative
-// when they live under the federation file's directory tree, absolute
-// otherwise — Save As recomputes against the new location.
-//
-// Round-trip-only fields today (no UI to edit, but preserved across load/
-// save): per-model `transform` (4x4, column-major), per-model `visible`,
-// future cloud `source.kind`s.
-class Federation : public QObject {
-    Q_OBJECT
-public:
-    struct HomeView {
-        QVector3D target;
-        float distance = 50.0f;
-        float yaw = 45.0f;     // degrees
-        float pitch = 30.0f;   // degrees
-    };
-
-    struct Model {
-        QString id;                       // stable, persisted
-        QString display_name;
-        QString source_kind = "local";    // future: "http", "speckle", ...
-        QString source_path;              // resolved absolute when kind == "local"
-        bool has_transform = false;
-        std::array<float, 16> transform{}; // column-major; identity when !has_transform
-        bool visible = true;
-    };
-
-    explicit Federation(QObject* parent = nullptr);
-
-    // Round-trip
-    bool load(const QString& path, QStringList* warnings, QString* err);
-    bool save(const QString& path, QString* err);
-
-    // Mutations
-    void clear();
-    QString addModel(const QString& source_path,
-                     const QString& display_name = QString());
-    void removeModel(const QString& fed_id);
-    void setHomeView(const HomeView& hv);
-    void clearHomeView();
-
-    // Accessors
-    const std::vector<Model>& models() const { return models_; }
-    const Model* findById(const QString& fed_id) const;
-    bool isDirty() const { return dirty_; }
-    void markClean();
-    QString filePath() const { return file_path_; }
-    QString name() const { return name_; }
-    bool hasHomeView() const { return has_home_view_; }
-    const HomeView& homeView() const { return home_view_; }
-
-signals:
-    void dirtyChanged(bool dirty);
-
-private:
-    void setDirty(bool d);
-    static QString generateId();
-    static bool isFederationPath(const QString& path);
-
-    QString file_path_;
-    QString name_;
-    QDateTime created_;
-    QDateTime modified_;
-    std::vector<Model> models_;
-    bool has_home_view_ = false;
-    HomeView home_view_;
-    bool dirty_ = false;
-};
-
-#endif // FEDERATION_H
diff --git a/src/ifcviewer-full/tests/CMakeLists.txt b/src/ifcviewer-full/tests/CMakeLists.txt
deleted file mode 100644
index 705fec1ad0..0000000000
--- a/src/ifcviewer-full/tests/CMakeLists.txt
+++ /dev/null
@@ -1,41 +0,0 @@
-################################################################################
-#                                                                              #
-# This file is part of IfcOpenShell.                                           #
-#                                                                              #
-# IfcOpenShell is free software: you can redistribute it and/or modify         #
-# it under the terms of the Lesser GNU General Public License as published by  #
-# the Free Software Foundation, either version 3.0 of the License, or          #
-# (at your option) any later version.                                          #
-#                                                                              #
-# IfcOpenShell is distributed in the hope that it will be useful,              #
-# but WITHOUT ANY WARRANTY; without even the implied warranty of               #
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                 #
-# Lesser GNU General Public License for more details.                          #
-#                                                                              #
-# You should have received a copy of the Lesser GNU General Public License     #
-# along with this program. If not, see <http://www.gnu.org/licenses/>.         #
-#                                                                              #
-################################################################################
-
-# Tier-1 tests for ifcviewer-full. Federation is QObject-derived but only
-# uses Qt6::Core (no event loop, no GL), so tests can construct it directly.
-
-set(IFCVIEWER_FULL_SRC ${CMAKE_CURRENT_SOURCE_DIR}/..)
-
-# Federation::HomeView holds a QVector3D (defined in QtGui), and QSignalSpy
-# / QTest live in Qt6::Test.
-find_package(Qt${QT_VERSION} COMPONENTS Core Gui Test REQUIRED PATHS ${QT_DIR})
-
-add_executable(test_federation
-    test_federation.cpp
-    ${IFCVIEWER_FULL_SRC}/Federation.cpp
-)
-set_target_properties(test_federation PROPERTIES AUTOMOC ON)
-target_include_directories(test_federation PRIVATE ${IFCVIEWER_FULL_SRC})
-target_link_libraries(test_federation PRIVATE
-    Catch2::Catch2WithMain
-    Qt${QT_VERSION}::Core
-    Qt${QT_VERSION}::Gui    # Federation::HomeView uses QVector3D from QtGui
-    Qt${QT_VERSION}::Test   # QSignalSpy
-)
-catch_discover_tests(test_federation)
diff --git a/src/ifcviewer/Federation.cpp b/src/ifcviewer/Federation.cpp
index 12ba3acb0f..166a48910d 100644
--- a/src/ifcviewer/Federation.cpp
+++ b/src/ifcviewer/Federation.cpp
@@ -18,86 +18,275 @@
  ********************************************************************************/
 
 #include "Federation.h"
-#include "Unit.h"
 
-#include <cmath>
+#include <QDir>
+#include <QFile>
+#include <QFileInfo>
+#include <QSaveFile>
+#include <QJsonArray>
+#include <QJsonDocument>
+#include <QJsonObject>
+#include <QJsonValue>
+#include <QUuid>
 
 namespace {
+constexpr const char* kSchema = "ifcfed/1";
 
-constexpr double kPi       = 3.14159265358979323846;
-constexpr double kDegToRad = kPi / 180.0;
-
-Eigen::Matrix4d translation4(const Eigen::Vector3d& t) {
-    Eigen::Matrix4d M = Eigen::Matrix4d::Identity();
-    M(0, 3) = t.x();
-    M(1, 3) = t.y();
-    M(2, 3) = t.z();
-    return M;
+QString resolvePath(const QString& fed_dir, const QString& stored) {
+    if (stored.isEmpty()) return stored;
+    QFileInfo fi(stored);
+    if (fi.isAbsolute()) return QDir::cleanPath(stored);
+    return QDir::cleanPath(QDir(fed_dir).absoluteFilePath(stored));
 }
 
-// Intrinsic XYZ Euler: R = R_z(z) · R_y(y) · R_x(x).
-Eigen::Matrix4d eulerXYZ(const Eigen::Vector3d& rxyz_rad) {
-    const Eigen::Matrix3d R3 =
-        (Eigen::AngleAxisd(rxyz_rad.z(), Eigen::Vector3d::UnitZ()) *
-         Eigen::AngleAxisd(rxyz_rad.y(), Eigen::Vector3d::UnitY()) *
-         Eigen::AngleAxisd(rxyz_rad.x(), Eigen::Vector3d::UnitX())).matrix();
-    Eigen::Matrix4d R = Eigen::Matrix4d::Identity();
-    R.block<3, 3>(0, 0) = R3;
-    return R;
+// Returns abs_path relative to fed_dir if abs_path lives under fed_dir,
+// otherwise returns abs_path unchanged.
+QString relativizePath(const QString& fed_dir, const QString& abs_path) {
+    QString fed_canon = QDir::cleanPath(fed_dir);
+    QString abs_canon = QDir::cleanPath(abs_path);
+    if (!fed_canon.endsWith('/')) fed_canon += '/';
+    if (abs_canon.startsWith(fed_canon)) {
+        return QDir(fed_canon).relativeFilePath(abs_canon);
+    }
+    return abs_canon;
 }
-
 }  // namespace
 
-double federationUnitToMeters(const FederationConfig& cfg) {
-    return convert(1.0, cfg.unit_prefix, cfg.unit_name, "", "METRE");
+Federation::Federation(QObject* parent) : QObject(parent) {}
+
+QString Federation::generateId() {
+    return QUuid::createUuid().toString(QUuid::WithoutBraces);
 }
 
-Eigen::Matrix4d composeFederationOrigin(const FederationOrigin& origin,
-                                        const FederationConfig& cfg) {
-    const double u = federationUnitToMeters(cfg);
-    const Eigen::Vector3d xyz_m = origin.xyz * u;
-    const double rz_rad = origin.rz_deg * kDegToRad;
-
-    const Eigen::Matrix3d Rz =
-        Eigen::AngleAxisd(rz_rad, Eigen::Vector3d::UnitZ()).matrix();
-    Eigen::Matrix4d Rz4 = Eigen::Matrix4d::Identity();
-    Rz4.block<3, 3>(0, 0) = Rz;
-
-    return Rz4 * translation4(-xyz_m);
+bool Federation::isFederationPath(const QString& path) {
+    return path.endsWith(".ifcfed", Qt::CaseInsensitive);
 }
 
-Eigen::Matrix4d composeModelTransform(const ModelTransform& xf,
-                                      const FederationConfig& fed_cfg,
-                                      const ModelUnits& model_units,
-                                      const Eigen::Matrix4d& stage2_meters) {
-    const double u_fed = federationUnitToMeters(fed_cfg);
+void Federation::clear() {
+    file_path_.clear();
+    name_.clear();
+    created_ = QDateTime();
+    modified_ = QDateTime();
+    models_.clear();
+    has_home_view_ = false;
+    home_view_ = HomeView{};
+    setDirty(false);
+}
 
-    Eigen::Vector3d A_m;
-    if (xf.a_frame == AFrame::ModelLocal) {
-        // a is in the model's project length unit, expressed in the
-        // pre-stage2 frame.  Convert to metres, then lift through stage 2.
-        const Eigen::Vector4d a_h(
-            xf.a.x() * model_units.project_length_to_meters,
-            xf.a.y() * model_units.project_length_to_meters,
-            xf.a.z() * model_units.project_length_to_meters,
-            1.0);
-        A_m = (stage2_meters * a_h).head<3>();
-    } else {
-        // a is in the model's map unit, expressed in the post-stage2 frame.
-        A_m = xf.a * model_units.map_unit_to_meters;
+void Federation::markClean() {
+    setDirty(false);
+}
+
+void Federation::setDirty(bool d) {
+    if (dirty_ == d) return;
+    dirty_ = d;
+    emit dirtyChanged(d);
+}
+
+const Federation::Model* Federation::findById(const QString& fed_id) const {
+    for (const auto& m : models_) {
+        if (m.id == fed_id) return &m;
+    }
+    return nullptr;
+}
+
+QString Federation::addModel(const QString& source_path,
+                              const QString& display_name) {
+    if (source_path.isEmpty()) return {};
+    if (isFederationPath(source_path)) return {};  // no nested federations
+
+    Model m;
+    m.id = generateId();
+    m.display_name = display_name.isEmpty()
+        ? QFileInfo(source_path).fileName()
+        : display_name;
+    m.source_kind = "local";
+    m.source_path = QDir::cleanPath(QFileInfo(source_path).absoluteFilePath());
+    models_.push_back(std::move(m));
+    setDirty(true);
+    return models_.back().id;
+}
+
+void Federation::removeModel(const QString& fed_id) {
+    for (auto it = models_.begin(); it != models_.end(); ++it) {
+        if (it->id == fed_id) {
+            models_.erase(it);
+            setDirty(true);
+            return;
+        }
+    }
+}
+
+void Federation::setHomeView(const HomeView& hv) {
+    home_view_ = hv;
+    has_home_view_ = true;
+    setDirty(true);
+}
+
+void Federation::clearHomeView() {
+    if (!has_home_view_) return;
+    has_home_view_ = false;
+    home_view_ = HomeView{};
+    setDirty(true);
+}
+
+bool Federation::load(const QString& path,
+                      QStringList* warnings,
+                      QString* err) {
+    QFile f(path);
+    if (!f.open(QIODevice::ReadOnly)) {
+        if (err) *err = QString("Cannot open %1: %2").arg(path, f.errorString());
+        return false;
+    }
+    QByteArray bytes = f.readAll();
+    f.close();
+
+    QJsonParseError pe;
+    QJsonDocument doc = QJsonDocument::fromJson(bytes, &pe);
+    if (doc.isNull() || !doc.isObject()) {
+        if (err) *err = QString("Parse error in %1: %2").arg(path, pe.errorString());
+        return false;
+    }
+    QJsonObject root = doc.object();
+
+    clear();
+    file_path_ = QDir::cleanPath(QFileInfo(path).absoluteFilePath());
+    QString fed_dir = QFileInfo(file_path_).absolutePath();
+
+    QString schema = root.value("schema").toString();
+    if (schema != kSchema && warnings) {
+        *warnings << QString("Unknown schema '%1' (expected '%2'); attempting to load anyway.")
+                         .arg(schema, kSchema);
     }
 
-    const Eigen::Vector3d B_m     = xf.b     * u_fed;
-    const Eigen::Vector3d pivot_m = xf.pivot * u_fed;
+    name_ = root.value("name").toString();
+    created_ = QDateTime::fromString(root.value("created").toString(), Qt::ISODate);
+    modified_ = QDateTime::fromString(root.value("modified").toString(), Qt::ISODate);
 
-    const Eigen::Matrix4d R_local = eulerXYZ(xf.rxyz_deg * kDegToRad);
-    const Eigen::Matrix4d R_at_pivot =
-        translation4(pivot_m) * R_local * translation4(-pivot_m);
+    QJsonArray arr = root.value("models").toArray();
+    for (int i = 0; i < arr.size(); ++i) {
+        if (!arr[i].isObject()) {
+            if (warnings) *warnings << QString("models[%1] is not an object; skipping.").arg(i);
+            continue;
+        }
+        QJsonObject mo = arr[i].toObject();
 
-    // Translate so R_at_pivot · A lands at B.
-    const Eigen::Vector4d Ah(A_m.x(), A_m.y(), A_m.z(), 1.0);
-    const Eigen::Vector3d RA = (R_at_pivot * Ah).head<3>();
-    const Eigen::Matrix4d T  = translation4(B_m - RA);
+        Model m;
+        m.id = mo.value("id").toString();
+        if (m.id.isEmpty()) m.id = generateId();
+        m.display_name = mo.value("display_name").toString();
 
-    return T * R_at_pivot;
+        QJsonObject so = mo.value("source").toObject();
+        m.source_kind = so.value("kind").toString("local");
+        if (m.source_kind != "local") {
+            if (warnings)
+                *warnings << QString("models[%1]: unsupported source kind '%2'; entry kept but not loaded.")
+                                 .arg(i).arg(m.source_kind);
+            // Keep raw stored path so save() round-trips correctly.
+            m.source_path = so.value("path").toString();
+            models_.push_back(std::move(m));
+            continue;
+        }
+
+        QString stored = so.value("path").toString();
+        if (stored.isEmpty()) {
+            if (warnings) *warnings << QString("models[%1]: missing source.path; skipping.").arg(i);
+            continue;
+        }
+        m.source_path = resolvePath(fed_dir, stored);
+
+        if (m.display_name.isEmpty())
+            m.display_name = QFileInfo(m.source_path).fileName();
+
+        QJsonValue vv = mo.value("visible");
+        if (vv.isBool()) m.visible = vv.toBool();
+
+        models_.push_back(std::move(m));
+    }
+
+    QJsonValue hv = root.value("home_view");
+    if (hv.isObject()) {
+        QJsonObject ho = hv.toObject();
+        QJsonArray ta = ho.value("target").toArray();
+        HomeView v;
+        if (ta.size() == 3) {
+            v.target = QVector3D(float(ta[0].toDouble()),
+                                 float(ta[1].toDouble()),
+                                 float(ta[2].toDouble()));
+        }
+        v.distance = float(ho.value("distance").toDouble(50.0));
+        v.yaw      = float(ho.value("yaw").toDouble(45.0));
+        v.pitch    = float(ho.value("pitch").toDouble(30.0));
+        home_view_ = v;
+        has_home_view_ = true;
+    }
+
+    setDirty(false);
+    return true;
+}
+
+bool Federation::save(const QString& path, QString* err) {
+    QString abs_path = QDir::cleanPath(QFileInfo(path).absoluteFilePath());
+    QString fed_dir = QFileInfo(abs_path).absolutePath();
+
+    QJsonObject root;
+    root["schema"] = kSchema;
+    if (!name_.isEmpty()) root["name"] = name_;
+
+    if (!created_.isValid()) created_ = QDateTime::currentDateTimeUtc();
+    modified_ = QDateTime::currentDateTimeUtc();
+    root["created"]  = created_.toUTC().toString(Qt::ISODate);
+    root["modified"] = modified_.toUTC().toString(Qt::ISODate);
+
+    QJsonArray arr;
+    for (const auto& m : models_) {
+        QJsonObject mo;
+        mo["id"] = m.id;
+        mo["display_name"] = m.display_name;
+
+        QJsonObject so;
+        so["kind"] = m.source_kind;
+        if (m.source_kind == "local") {
+            so["path"] = relativizePath(fed_dir, m.source_path);
+        } else {
+            // Round-trip raw value for unsupported kinds.
+            so["path"] = m.source_path;
+        }
+        mo["source"] = so;
+
+        if (!m.visible) mo["visible"] = false;
+
+        arr.append(mo);
+    }
+    root["models"] = arr;
+
+    if (has_home_view_) {
+        QJsonObject ho;
+        QJsonArray ta;
+        ta.append(double(home_view_.target.x()));
+        ta.append(double(home_view_.target.y()));
+        ta.append(double(home_view_.target.z()));
+        ho["target"]   = ta;
+        ho["distance"] = double(home_view_.distance);
+        ho["yaw"]      = double(home_view_.yaw);
+        ho["pitch"]    = double(home_view_.pitch);
+        root["home_view"] = ho;
+    } else {
+        root["home_view"] = QJsonValue();  // null
+    }
+
+    QSaveFile f(abs_path);
+    if (!f.open(QIODevice::WriteOnly | QIODevice::Truncate)) {
+        if (err) *err = QString("Cannot write %1: %2").arg(abs_path, f.errorString());
+        return false;
+    }
+    f.write(QJsonDocument(root).toJson(QJsonDocument::Indented));
+    if (!f.commit()) {
+        if (err) *err = QString("Failed to commit %1: %2").arg(abs_path, f.errorString());
+        return false;
+    }
+
+    file_path_ = abs_path;
+    setDirty(false);
+    return true;
 }
diff --git a/src/ifcviewer/Federation.h b/src/ifcviewer/Federation.h
index 885ee51ed9..f3174582cc 100644
--- a/src/ifcviewer/Federation.h
+++ b/src/ifcviewer/Federation.h
@@ -17,100 +17,85 @@
  *                                                                              *
  ********************************************************************************/
 
-// Federation-level transformation data model and compose helpers.
-//
-// A federation is the user's working scene, composed of one or more IFC
-// models.  Each model lives at:
-//
-//     stage3 · stage4 · stage2 · placement_stage1
-//
-// where:
-//   - stage1 is per-mesh vertex rebasing (applied to the geometry buffers)
-//   - stage2 is the per-model georef matrix (immutable, derived from the IFC)
-//   - stage3 is the federation-wide false origin (mutable, this header)
-//   - stage4 is the per-model placement within the federation (mutable, this header)
-//
-// All composed matrices are in metres.  The user-authored intent is stored
-// in source units (model project unit / model map unit / federation unit) to
-// preserve precision; conversion to metres happens in the compose helpers.
-
 #ifndef FEDERATION_H
 #define FEDERATION_H
 
-#include <Eigen/Dense>
+#include <QObject>
+#include <QString>
+#include <QStringList>
+#include <QDateTime>
+#include <QVector3D>
 
-#include <string>
+#include <vector>
 
-// Federation-wide settings persisted in .ifcfed.
-struct FederationConfig {
-    // IfcSIUnit name ("METRE") or IfcConversionBasedUnit name ("foot", "inch", ...).
-    std::string unit_name   = "METRE";
-    // SI prefix ("MILLI", "KILO", ...) — empty for unprefixed or for
-    // conversion-based units.
-    std::string unit_prefix = "";
-};
-
-// Stage 3 — the federation false origin.  Authoring intent is "nominate this
-// XYZ as the new origin, with optional Z-axis heading rotation".  Composed as
+// In-memory representation of an .ifcfed file (IFC federation).
 //
-//     stage3 = R_z(rz_deg) · T(-xyz_in_metres)
+// A federation is a named, ordered list of model sources plus an optional
+// "home view" camera state.  Source paths can be relative (resolved against
+// the .ifcfed's directory) or absolute.  Save() reserialises paths relative
+// when they live under the federation file's directory tree, absolute
+// otherwise — Save As recomputes against the new location.
 //
-// i.e. translate the federation so the nominated point lands at the origin,
-// then rotate around the new origin.  Translation is given in federation unit;
-// rotation is in degrees.
-struct FederationOrigin {
-    Eigen::Vector3d xyz    = Eigen::Vector3d::Zero();   // federation unit
-    double          rz_deg = 0.0;                       // degrees
+// Round-trip-only fields today (no UI to edit, but preserved across load/
+// save): per-model `visible`, future cloud `source.kind`s.
+class Federation : public QObject {
+    Q_OBJECT
+public:
+    struct HomeView {
+        QVector3D target;
+        float distance = 50.0f;
+        float yaw = 45.0f;     // degrees
+        float pitch = 30.0f;   // degrees
+    };
+
+    struct Model {
+        QString id;                       // stable, persisted
+        QString display_name;
+        QString source_kind = "local";    // future: "http", "speckle", ...
+        QString source_path;              // resolved absolute when kind == "local"
+        bool visible = true;
+    };
+
+    explicit Federation(QObject* parent = nullptr);
+
+    // Round-trip
+    bool load(const QString& path, QStringList* warnings, QString* err);
+    bool save(const QString& path, QString* err);
+
+    // Mutations
+    void clear();
+    QString addModel(const QString& source_path,
+                     const QString& display_name = QString());
+    void removeModel(const QString& fed_id);
+    void setHomeView(const HomeView& hv);
+    void clearHomeView();
+
+    // Accessors
+    const std::vector<Model>& models() const { return models_; }
+    const Model* findById(const QString& fed_id) const;
+    bool isDirty() const { return dirty_; }
+    void markClean();
+    QString filePath() const { return file_path_; }
+    QString name() const { return name_; }
+    bool hasHomeView() const { return has_home_view_; }
+    const HomeView& homeView() const { return home_view_; }
+
+signals:
+    void dirtyChanged(bool dirty);
+
+private:
+    void setDirty(bool d);
+    static QString generateId();
+    static bool isFederationPath(const QString& path);
+
+    QString file_path_;
+    QString name_;
+    QDateTime created_;
+    QDateTime modified_;
+    std::vector<Model> models_;
+    bool has_home_view_ = false;
+    HomeView home_view_;
+    bool dirty_ = false;
 };
 
-// Frame in which ModelTransform.a is expressed.
-//   ModelLocal  — pre-stage2 model coordinates, in the model's project length unit
-//   ModelGlobal — post-stage2 model coordinates, in the model's map unit
-enum class AFrame { ModelLocal, ModelGlobal };
-
-// Stage 4 — the per-model placement within the federation.  Authoring intent
-// is "rotate the model around `pivot`, then translate so that point `a` lands
-// at point `b`".  Composed as
-//
-//     R_local    = R_z(rz) · R_y(ry) · R_x(rx)            [intrinsic XYZ]
-//     R_at_pivot = T(pivot_m) · R_local · T(-pivot_m)
-//     stage4     = T(b_m - R_at_pivot · a_m) · R_at_pivot
-//
-// Numbers are stored in their original input unit (a in model project or map
-// unit per a_frame, b/pivot in federation unit) so that the user's typed
-// values round-trip without precision loss.
-struct ModelTransform {
-    AFrame          a_frame  = AFrame::ModelGlobal;
-    Eigen::Vector3d a        = Eigen::Vector3d::Zero();   // model project / map unit
-    Eigen::Vector3d b        = Eigen::Vector3d::Zero();   // federation unit
-    Eigen::Vector3d rxyz_deg = Eigen::Vector3d::Zero();   // degrees, intrinsic XYZ
-    Eigen::Vector3d pivot    = Eigen::Vector3d::Zero();   // federation unit
-};
-
-// Per-model unit scales captured at load time.  project_length_to_meters
-// comes from calculateUnitScale(file, "LENGTHUNIT"); map_unit_to_meters from
-// siScaleFromNamedUnit(getMapUnit(file)) and falls back to the project length
-// scale when the model has no MapUnit.
-struct ModelUnits {
-    double project_length_to_meters = 1.0;
-    double map_unit_to_meters       = 1.0;
-};
-
-// 1 federation_unit -> N metres.  Cached at the call site if needed.
-double federationUnitToMeters(const FederationConfig&);
-
-// Compose stage 3 (federation false origin) into a 4x4 matrix in metres.
-Eigen::Matrix4d composeFederationOrigin(const FederationOrigin&,
-                                        const FederationConfig&);
-
-// Compose stage 4 (per-model placement within the federation) into a 4x4
-// matrix in metres.  `stage2_meters` is the model's georef matrix (e.g.
-// helmertMetersFromParameters · inv(wcs_meters)) — needed to lift `a` into
-// metres when a_frame == ModelLocal.  Pass identity when stage 2 is disabled
-// or absent.
-Eigen::Matrix4d composeModelTransform(const ModelTransform&,
-                                      const FederationConfig& fed_cfg,
-                                      const ModelUnits& model_units,
-                                      const Eigen::Matrix4d& stage2_meters);
-
 #endif // FEDERATION_H
diff --git a/src/ifcviewer/tests/CMakeLists.txt b/src/ifcviewer/tests/CMakeLists.txt
index d6739bb22b..70436a9b73 100644
--- a/src/ifcviewer/tests/CMakeLists.txt
+++ b/src/ifcviewer/tests/CMakeLists.txt
@@ -47,3 +47,22 @@ add_ifcviewer_unit_test(test_sidecar_cache
 )
 
 add_ifcviewer_unit_test(test_instanced_geometry)
+
+# Federation is Qt-derived (QObject + signals + QVector3D + QJson*).  Unlike
+# the other Tier-1 tests it has to pull Qt6::Core/Gui/Test in directly and
+# enable AUTOMOC for the Q_OBJECT moc-generation.
+find_package(Qt${QT_VERSION} COMPONENTS Core Gui Test REQUIRED PATHS ${QT_DIR})
+
+add_executable(test_federation
+    test_federation.cpp
+    ${IFCVIEWER_SRC}/Federation.cpp
+)
+set_target_properties(test_federation PROPERTIES AUTOMOC ON)
+target_include_directories(test_federation PRIVATE ${IFCVIEWER_SRC})
+target_link_libraries(test_federation PRIVATE
+    Catch2::Catch2WithMain
+    Qt${QT_VERSION}::Core
+    Qt${QT_VERSION}::Gui    # Federation::HomeView uses QVector3D from QtGui
+    Qt${QT_VERSION}::Test   # QSignalSpy
+)
+catch_discover_tests(test_federation)
diff --git a/src/ifcviewer-full/tests/test_federation.cpp b/src/ifcviewer/tests/test_federation.cpp
similarity index 100%
rename from src/ifcviewer-full/tests/test_federation.cpp
rename to src/ifcviewer/tests/test_federation.cpp