diff --git a/src/bonsai/bonsai/bim/module/material/operator.py b/src/bonsai/bonsai/bim/module/material/operator.py
index e0761bf416..7d8f9edd9d 100644
--- a/src/bonsai/bonsai/bim/module/material/operator.py
+++ b/src/bonsai/bonsai/bim/module/material/operator.py
@@ -614,25 +614,29 @@ class EditAssignedMaterial(bpy.types.Operator, tool.Ifc.Operator):
                     attributes=attributes,
                 )
 
-                layer_sets_to_regenerate = set()
+                slab_planer = slab.DumbSlabPlaner()
+                wall_objs = []
 
                 for obj in objects:
                     obj_element = tool.Ifc.get_entity(obj)
                     obj_material_usage = ifcopenshell.util.element.get_material(obj_element)
-
                     if obj_material_usage and obj_material_usage.is_a("IfcMaterialLayerSetUsage"):
                         obj_material_usage.OffsetFromReferenceLine = material.OffsetFromReferenceLine
                         obj_material_usage.DirectionSense = material.DirectionSense
                         obj_material_usage.ReferenceExtent = material.ReferenceExtent
 
-                        layer_sets_to_regenerate.add(obj_material_usage.ForLayerSet)
-
                         # Save custom offset to BBIM_MaterialLayer pset
                         tool.Model.save_custom_offset_to_pset(obj_element, obj)
 
-                for layer_set in layer_sets_to_regenerate:
-                    wall.DumbWallPlaner().regenerate_from_layer_set(layer_set)
-                    slab.DumbSlabPlaner().regenerate_from_layer_set(layer_set)
+                        # Targeted regeneration: only update this element's geometry, not
+                        # all elements sharing the layer set (which would corrupt unrelated instances).
+                        if obj_material_usage.LayerSetDirection == "AXIS3":
+                            slab_planer.regenerate_from_occurence(obj_element, obj_material_usage)
+                        elif obj_material_usage.LayerSetDirection == "AXIS2":
+                            wall_objs.append(obj)
+
+                if wall_objs:
+                    tool.Model.recalculate_walls(wall_objs)
 
             if material_set_usage.is_a("IfcMaterialProfileSetUsage"):
                 if "CardinalPoint" in attributes:
diff --git a/src/bonsai/bonsai/bim/module/model/slab.py b/src/bonsai/bonsai/bim/module/model/slab.py
index 58a353ab28..a2957048a9 100644
--- a/src/bonsai/bonsai/bim/module/model/slab.py
+++ b/src/bonsai/bonsai/bim/module/model/slab.py
@@ -280,6 +280,8 @@ class DumbSlabPlaner:
                 perpendicular_depth = thickness * abs(1 / cos(existing_x_angle))
                 perpendicular_offset = layer_offset * abs(1 / cos(existing_x_angle)) / self.unit_scale
 
+                ifc_position = extrusion.Position
+
                 # Check angle and z direction to determine whether the extrusion direction is positive or negative
                 if (abs(existing_x_angle) < (pi / 2) and direction_ratios.z > 0) or (
                     abs(existing_x_angle) > (pi / 2) and direction_ratios.z < 0
@@ -301,7 +303,6 @@ class DumbSlabPlaner:
                 extrusion.ExtrudedDirection.DirectionRatios = tuple(direction_ratios)
                 extrusion.Depth = perpendicular_depth
 
-                ifc_position = extrusion.Position
                 position = offset_direction * perpendicular_offset
                 material = ifcopenshell.util.element.get_material(element)
                 if material:
diff --git a/src/bonsai/bonsai/tool/loader.py b/src/bonsai/bonsai/tool/loader.py
index 6d30192485..8d11f79670 100644
--- a/src/bonsai/bonsai/tool/loader.py
+++ b/src/bonsai/bonsai/tool/loader.py
@@ -1063,12 +1063,16 @@ class Loader(bonsai.core.tool.Loader):
 
     @classmethod
     def slice_layerset_mesh(cls, element: ifcopenshell.entity_instance, mesh: bpy.types.Mesh) -> bpy.types.Mesh:
+        # Always compute unit_scale fresh — cls.unit_scale may be stale (e.g. during live
+        # geometry updates that don't go through the full import pipeline).
+        unit_scale = ifcopenshell.util.unit.calculate_unit_scale(tool.Ifc.get())
+
         if not (material := ifcopenshell.util.element.get_material(element)):
             return mesh
         elif material.is_a("IfcMaterialLayerSetUsage"):
             usage = material
             layer_set = material.ForLayerSet
-            offset = usage.OffsetFromReferenceLine * cls.unit_scale
+            offset = usage.OffsetFromReferenceLine * unit_scale
             sense_factor = 1 if usage.DirectionSense == "POSITIVE" else -1
         else:
             return mesh
@@ -1101,7 +1105,7 @@ class Loader(bonsai.core.tool.Loader):
         for i, layer in enumerate(layer_set.MaterialLayers):
             if i != last_i:
                 prev_co = co.copy()
-                co += no * layer.LayerThickness * cls.unit_scale
+                co += no * layer.LayerThickness * unit_scale
                 bisect_geom = bmesh.ops.bisect_plane(
                     bm, geom=bm.verts[:] + bm.edges[:] + bm.faces[:], dist=0.0001, plane_co=co, plane_no=no
                 )
@@ -1115,14 +1119,17 @@ class Loader(bonsai.core.tool.Loader):
                 for face in bisect_geom["geom"]:
                     if isinstance(face, bmesh.types.BMFace):
                         center = face.calc_center_median()
-                        if (center - co).dot(no) >= 0:
+                        dot = (center - co).dot(no)
+                        if dot >= 0:
                             face.material_index = material_index
                             has_layer_styles = True
             else:
                 for face in bisect_geom["geom"]:
                     if isinstance(face, bmesh.types.BMFace):
                         center = face.calc_center_median()
-                        if (center - co).dot(no) < 0 and (center - prev_co).dot(no) >= 0:
+                        dot_co = (center - co).dot(no)
+                        dot_prev = (center - prev_co).dot(no)
+                        if dot_co < 0 and dot_prev >= 0:
                             face.material_index = material_index
                             has_layer_styles = True