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+# Design Spec: Space Regeneration with Sloped Roofs, Walls, and Slabs
+
+## Goal
+
+Extend `bonsai.core.spatial.generate_space` so it produces correct `IfcSpace`
+geometry for non-rectilinear envelopes:
+
+- sloped roofs,
+- sloped slabs,
+- sloped walls,
+- curved walls.
+
+The existing footprint-based `IfcExtrudedAreaSolid` path is preserved for
+ordinary vertical extrusions. A new hybrid path keeps the representation
+parametric when possible and falls back to an `IfcFacetedBrep` only when the
+boundary cannot be expressed as a clipped extrusion.
+
+## Architecture
+
+```
+┌─────────────────────────────────────────┐
+│ Existing footprint generation            │
+│ (get_space_polygon_from_*_objects)     │
+└──────────────┬────────────────────────────┘
+               v
+┌─────────────────────────────────────────┐
+│ Detect extrudability and bounding planes │
+│ (pure-Python util, Blender-independent)  │
+└──────────────┬────────────────────────────┘
+               v
+        ┌──────┴──────┐
+        v             v
+┌───────────────────┐ ┌───────────────────┐
+│ Extrusion + clips │ │ B-rep fallback     │
+│ IfcExtrudedAreaSolid│ │ IfcFacetedBrep    │
+│ + IfcBooleanClippingResult│ │ (or IfcPolygonalFaceSet) │
+└───────────────────┘ └───────────────────┘
+```
+
+## Detection criteria
+
+Use the parametric `IfcExtrudedAreaSolid` + `IfcBooleanClippingResult` path
+when **all** are true:
+
+1. Side walls are vertical extrusions (face normal is horizontal).
+   Curved-in-plan walls are allowed; their footprint is polygonized or
+   reconstructed as a curved profile.
+2. The roof/top boundary is piecewise-planar.
+3. The bottom slab/floor boundary is piecewise-planar.
+4. The footprint is a single closed outer region, possibly with inner closed
+   regions for holes.
+5. The resulting half-space intersection is non-empty and produces a single
+   solid.
+
+Otherwise use the B-rep fallback.
+
+## Parametric extrusion + clipping algorithm
+
+1. **Build the profile**
+   - Outer ring from the footprint polygon → `IfcArbitraryClosedProfileDef`.
+   - Inner rings (holes, e.g., around columns) →
+     `IfcArbitraryProfileDefWithVoids`.
+2. **Extrude**
+   - Create `IfcExtrudedAreaSolid` along local +Z, with a height large enough
+     to cover all bounding planes.
+3. **Find top planes**
+   - Cast vertical rays upward from the footprint centroid and sample points
+     using `ifcopenshell.geom.tree.select_ray`.
+   - Check each hit face for planarity with
+     `ifcopenshell.util.shape.dissolve_faces(..., merge_coplanar=True)`.
+   - Group coplanar hits into distinct planes.
+4. **Find bottom planes**
+   - Same as top, but downward.
+5. **Clip**
+   - For each top plane: create `IfcHalfSpaceSolid` with normal pointing
+     upward (removed side), apply via `ifcopenshell.api.geometry.clip_solid`.
+   - For each bottom plane: create `IfcHalfSpaceSolid` with normal pointing
+     downward, apply via `clip_solid`.
+6. **Output**
+   - `IfcExtrudedAreaSolid` wrapped in a chain of `IfcBooleanClippingResult`.
+
+## B-rep fallback algorithm
+
+For non-extrudable cases (sloped walls, curved roofs, etc.):
+
+1. **Seed space**
+   - Create a temporary rough mesh (e.g., extruded footprint bounding box) as
+     a placeholder.
+2. **Extract boundary faces**
+   - Run `ifcopenshell.util.boundary.auto_generate_boundaries` against the
+     seed to identify the faces of bounding elements that touch the space.
+   - Convert each boundary polygon from face-local back to 3D world
+     coordinates.
+3. **Build closed shell**
+   - Collect the 3D boundary faces.
+   - Add narrow gap-closing faces if `auto_generate_boundaries` leaves
+     unmatched edges.
+   - Triangulate and produce `IfcClosedShell` → `IfcFacetedBrep` (or
+     `IfcPolygonalFaceSet` for IFC4+).
+4. **Clean up**
+   - Assign the B-rep to the `IfcSpace` and remove the temporary seed
+     geometry.
+
+## Files to touch
+
+- `src/ifcopenshell-python/ifcopenshell/util/space.py`
+  - New: `detect_space_volume_strategy`
+  - New: `build_extruded_clipped_space`
+  - New: `build_brep_space`
+  - New helpers for ray-cast plane detection and face planarity checks.
+- `src/bonsai/bonsai/tool/spatial.py`
+  - Extend `set_space_representation_from_polygon` to dispatch to the new
+    strategy.
+  - Extend footprint/profile creation to support inner rings for holes.
+- `src/bonsai/bonsai/core/spatial.py`
+  - `generate_space` calls the dispatcher.
+
+## Testing
+
+- Add unit tests in `src/ifcopenshell-python/test/util/test_space.py` for pure
+  geometry helpers:
+  - simple shed roof,
+  - gable roof,
+  - sloped slab,
+  - L-shaped footprint with sloped roof,
+  - curved wall.
+- Add Bonsai tests in `src/bonsai/test/tool/test_spatial.py` for end-to-end
+  `generate_space` with non-rectilinear geometry.
+
+## Error handling
+
+- If detection fails or half-space clipping produces an invalid result, fall
+  back to the B-rep path.
+- If the B-rep path also fails, return an error string and leave the existing
+  space representation unchanged.