IfcOpenShell · BIMvoice · Jul 20, 2026
diff --git a/src/bonsai/bonsai/bim/data/assets/default.css b/src/bonsai/bonsai/bim/data/assets/default.css
@@ -101,3 +101,20 @@ text.GRID, tspan.GRID { /* 5mm */ font-size: 8.25px; }
 .PredefinedType-STEEL { fill: url(#steel); stroke: black; stroke-width: 0.5; }
 .PredefinedType-CONCRETE { fill: url(#concrete); stroke: black; stroke-width: 0.5; }
 .PredefinedType-PLASTERBOARD { fill: url(#sand); stroke: black; stroke-width: 0.25; }
+/* Per element classes, assigned via EPset_Annotation.Classes. Declared last so
+   they override the class, material and predefined type rules above. */
+.fine { stroke-width: 0.18; }
+.thin { stroke-width: 0.25; }
+.medium { stroke-width: 0.35; }
+.thick { stroke-width: 0.5; }
+.strong { stroke-width: 1; }
+.dashed { stroke-dasharray: 3, 2; }
+.dotted { stroke-dasharray: 0.5, 1.5; }
+.dashdot { stroke-dasharray: 6, 2, 1, 2; }
+.hidden { stroke: none; fill: none; }
+.fill-none { fill: none; }
+.fill-white { fill: #ffffff; }
+.fill-light { fill: #dddddd; }
+.fill-grey { fill: #aaaaaa; }
+.fill-dark { fill: #777777; }
+.fill-solid { fill: #000000; }
diff --git a/src/bonsai/bonsai/bim/module/drawing/__init__.py b/src/bonsai/bonsai/bim/module/drawing/__init__.py
@@ -33,6 +33,7 @@
     operator.AddAnnotation,
     operator.AddAnnotationType,
     operator.AddDrawing,
+    operator.AddElementDrawingClass,
     operator.AddDrawingStyle,
     operator.AddDrawingToSheet,
     operator.AddReference,
@@ -76,6 +77,7 @@
     operator.SelectElementValues,
     operator.InsertFormattedLiteralPopup,
     operator.AddElementValueRow,
+    operator.RemoveElementDrawingClass,
     operator.RemoveElementValueRow,
     operator.ElementValueSuggestionsPopup,
     operator.FormatElementValueRow,
@@ -125,12 +127,14 @@
     ui.BIM_PT_sheets,
     ui.BIM_PT_drawings,
     ui.BIM_PT_camera,
+    ui.BIM_PT_element_drawing_classes,
     ui.BIM_PT_element_filters,
     ui.BIM_PT_drawing_underlay,
     ui.BIM_PT_schedules,
     ui.BIM_PT_references,
     ui.BIM_PT_product_assignments,
     ui.BIM_PT_text,
+    ui.BIM_MT_element_drawing_classes,
     ui.BIM_UL_drawinglist,
     ui.BIM_UL_sheets,
     # Core gizmos (shared across modules)

diff --git a/src/bonsai/bonsai/bim/module/drawing/data.py b/src/bonsai/bonsai/bim/module/drawing/data.py
@@ -39,6 +39,7 @@ def refresh():
     ElementFiltersData.is_loaded = False
     AnnotationData.is_loaded = False
     DecoratorData.is_loaded = False
+    ElementClassesData.is_loaded = False
 
 
 class ProductAssignmentsData:
@@ -231,6 +232,32 @@ def count_documents(cls):
     "title": 7.0,
 }
 
+# CSS classes shipped in default.css that may be assigned per element.
+# Custom stylesheets are free to define their own, so this is only a
+# convenience list for the UI, never a restriction.
+ELEMENT_CLASSES = {
+    "Line Weight": ("fine", "thin", "medium", "thick", "strong"),
+    "Line Style": ("dashed", "dotted", "dashdot", "hidden"),
+    "Fill": ("fill-none", "fill-white", "fill-light", "fill-grey", "fill-dark", "fill-solid"),
+}
+
+
+class ElementClassesData:
+    data = {}
+    is_loaded = False
+
+    @classmethod
+    def load(cls):
+        cls.data = {"classes": cls.classes()}
+        cls.is_loaded = True
+
+    @classmethod
+    def classes(cls) -> list[str]:
+        obj = bpy.context.active_object
+        if not obj or not (element := tool.Ifc.get_entity(obj)):
+            return []
+        return tool.Drawing.get_element_classes(element)
+
 
 class DecoratorData:
     # stores 1 type of data per object

diff --git a/src/bonsai/bonsai/bim/module/drawing/operator.py b/src/bonsai/bonsai/bim/module/drawing/operator.py
@@ -1480,6 +1480,9 @@ def get_svg_classes(self, element, layer=None):
                     tool.Drawing.canonicalise_class_name(key) + "-" + tool.Drawing.canonicalise_class_name(str(value))
                 )
 
+        # ─── Custom ────────────────────────────────────────────────
+        classes.extend(tool.Drawing.get_element_classes(element))
+
         return classes
 
     def is_manifold(self, obj) -> bool:
@@ -3647,6 +3650,53 @@ def _execute(self, context):
         core.disable_editing_assigned_product(tool.Drawing, obj=context.active_object)
 
 
+class AddElementDrawingClass(bpy.types.Operator, tool.Ifc.Operator):
+    bl_idname = "bim.add_element_drawing_class"
+    bl_label = "Add Drawing Class"
+    bl_description = (
+        "Assign a CSS class to this element so it can be styled individually in drawings.\n"
+        "Applies to all selected IFC objects"
+    )
+    bl_options = {"REGISTER", "UNDO"}
+
+    name: bpy.props.StringProperty(name="Class", default="")
+
+    if TYPE_CHECKING:
+        name: str
+
+    def invoke(self, context, event):
+        if self.name:
+            return self.execute(context)
+        return context.window_manager.invoke_props_dialog(self)
+
+    def _execute(self, context):
+        name = tool.Drawing.sanitise_class_name(self.name)
+        if not name:
+            self.report({"ERROR"}, "A valid CSS class name is required.")
+            return {"CANCELLED"}
+        for obj in tool.Blender.get_selected_objects():
+            if element := tool.Ifc.get_entity(obj):
+                core.add_element_class(tool.Drawing, element=element, name=name)
+        self.name = ""
+
+
+class RemoveElementDrawingClass(bpy.types.Operator, tool.Ifc.Operator):
+    bl_idname = "bim.remove_element_drawing_class"
+    bl_label = "Remove Drawing Class"
+    bl_description = "Unassign this CSS class from all selected IFC objects"
+    bl_options = {"REGISTER", "UNDO"}
+
+    name: bpy.props.StringProperty(name="Class")
+
+    if TYPE_CHECKING:
+        name: str
+
+    def _execute(self, context):
+        for obj in tool.Blender.get_selected_objects():
+            if element := tool.Ifc.get_entity(obj):
+                core.remove_element_class(tool.Drawing, element=element, name=self.name)
+
+
 class LoadSheets(bpy.types.Operator, tool.Ifc.Operator):
     bl_idname = "bim.load_sheets"
     bl_label = "Load Sheets"

diff --git a/src/bonsai/bonsai/bim/module/drawing/svgwriter.py b/src/bonsai/bonsai/bim/module/drawing/svgwriter.py
@@ -537,9 +537,7 @@ def get_attribute_classes(self, obj: bpy.types.Object) -> list[str]:
             str(ifcopenshell.util.element.get_predefined_type(element))
         )
         classes = [global_id, element.is_a(), predefined_type]
-        custom_classes: str = ifcopenshell.util.element.get_pset(element, "EPset_Annotation", "Classes")
-        if custom_classes:
-            classes.extend(custom_classes.split())
+        classes.extend(tool.Drawing.get_element_classes(element))
         for key in self.metadata:
             value = ifcopenshell.util.selector.get_element_value(element, key)
             if value:

diff --git a/src/bonsai/bonsai/bim/module/drawing/ui.py b/src/bonsai/bonsai/bim/module/drawing/ui.py
@@ -26,9 +26,11 @@
 import bonsai.bim.helper
 import bonsai.tool as tool
 from bonsai.bim.module.drawing.data import (
+    ELEMENT_CLASSES,
     DecoratorData,
     DocumentsData,
     DrawingsData,
+    ElementClassesData,
     ElementFiltersData,
     ProductAssignmentsData,
     SheetsData,
@@ -571,6 +573,55 @@ def draw(self, context):
             col.enabled = bool(ProductAssignmentsData.data["relating_product"])
 
 
+class BIM_MT_element_drawing_classes(bpy.types.Menu):
+    bl_label = "Add Drawing Class"
+    bl_idname = "BIM_MT_element_drawing_classes"
+
+    def draw(self, context):
+        assert self.layout
+        for category, names in ELEMENT_CLASSES.items():
+            self.layout.label(text=category)
+            for name in names:
+                self.layout.operator("bim.add_element_drawing_class", text=name).name = name
+            self.layout.separator()
+        self.layout.operator("bim.add_element_drawing_class", text="Custom Class...", icon="ADD").name = ""
+
+
+class BIM_PT_element_drawing_classes(Panel):
+    bl_label = "Drawing Classes"
+    bl_idname = "BIM_PT_element_drawing_classes"
+    bl_options = {"DEFAULT_CLOSED"}
+    bl_space_type = "PROPERTIES"
+    bl_region_type = "WINDOW"
+    bl_context = "object"
+    bl_order = 2
+    bl_parent_id = "BIM_PT_tab_object_metadata"
+
+    @classmethod
+    def poll(cls, context):
+        if not tool.Ifc.get() or not context.active_object:
+            return False
+        return bool(tool.Ifc.get_entity(context.active_object))
+
+    def draw(self, context):
+        if not ElementClassesData.is_loaded:
+            ElementClassesData.load()
+
+        assert self.layout
+        row = self.layout.row(align=True)
+        row.menu("BIM_MT_element_drawing_classes", icon="ADD", text="Add Class")
+
+        classes = ElementClassesData.data["classes"]
+        if not classes:
+            self.layout.label(text="No Classes Assigned", icon="BRUSH_DATA")
+            return
+
+        for name in classes:
+            row = self.layout.row(align=True)
+            row.label(text=name, icon="BRUSH_DATA")
+            row.operator("bim.remove_element_drawing_class", icon="X", text="").name = name
+
+
 def get_category_icon(category_name):
     """Get appropriate icon for each category"""
     icons = {

diff --git a/src/bonsai/bonsai/core/drawing.py b/src/bonsai/bonsai/core/drawing.py
@@ -637,3 +637,11 @@ def activate_drawing_view(
     blender.activate_camera(camera)
     drawing_tool.isolate_camera_collection(camera)
     drawing_tool.activate_drawing(camera)
+
+
+def add_element_class(drawing: type[tool.Drawing], element: ifcopenshell.entity_instance, name: str) -> None:
+    drawing.add_element_class(element, name)
+
+
+def remove_element_class(drawing: type[tool.Drawing], element: ifcopenshell.entity_instance, name: str) -> None:
+    drawing.remove_element_class(element, name)
diff --git a/src/bonsai/bonsai/core/tool.py b/src/bonsai/bonsai/core/tool.py
@@ -323,6 +323,7 @@ def update_document_objects(cls, document_id): pass
 @interface
 class Drawing:
     def activate_drawing(cls, camera): pass
+    def add_element_class(cls, element, name): pass
     def add_literal(cls, **attributes): pass
     def clear_annotation_relationships(cls, drawing): pass
     def copy_representation(cls, source, dest): pass
@@ -386,6 +387,7 @@ def get_drawing_document(cls, drawing): pass
     def get_drawing_group(cls, drawing): pass
     def get_drawing_references(cls, drawing): pass
     def get_drawing_target_view(cls, drawing): pass
+    def get_element_classes(cls, element): pass
     def get_group_drawing(cls, group): pass
     def get_group_elements(cls, group): pass
     def get_ifc_representation_class(cls, object_type): pass
@@ -414,20 +416,21 @@ def open_layout_svg(cls, uri): pass
     def open_spreadsheet(cls, uri): pass
     def open_svg(cls, filepath): pass
     def reload_representation(cls, obj, representation): pass
+    def remove_element_class(cls, element, name): pass
     def run_drawing_activate_model(cls): pass
     def run_root_assign_class(cls, obj=None, ifc_class=None, predefined_type=None, should_add_representation=True, context=None, ifc_representation_class=None): pass
     def run_type_assign_type(cls, element=None, relating_type=None): pass
+    def sanitise_class_name(cls, name): pass
     def select_assigned_product(cls, drawing): pass
     def set_camera_name(cls, drawing, name): pass
     def set_drawing_collection_name(cls, drawing, collection): pass
+    def set_element_classes(cls, element, classes): pass
     def set_name(cls, element, name): pass
     def setup_annotation_object(cls, obj, object_type): pass
     def setup_shading_styles_path(cls, resource_path): pass
     def show_decorations(cls): pass
     def sync_object_placement(cls, obj): pass
     def update_embedded_svg_location(cls, uri, old_location, new_location): pass
-
-
 @interface
 class Duplicate:
     def get_decomposition_relationships(cls, objs): pass

diff --git a/src/bonsai/bonsai/tool/drawing.py b/src/bonsai/bonsai/tool/drawing.py
@@ -2329,6 +2329,46 @@ def get_drawing_metadata(cls, drawing: ifcopenshell.entity_instance) -> list[str
         metadata_str = pset_data.get("Metadata", "") or ""
         return [v_ for v in metadata_str.split(",") if (v_ := v.strip())]
 
+    @classmethod
+    def get_element_classes(cls, element: ifcopenshell.entity_instance) -> list[str]:
+        classes = ifcopenshell.util.element.get_pset(element, "EPset_Annotation", "Classes")
+        if not isinstance(classes, str):
+            return []
+        return classes.split()
+
+    @classmethod
+    def set_element_classes(cls, element: ifcopenshell.entity_instance, classes: list[str]) -> None:
+        ifc_file = tool.Ifc.get()
+        pset = tool.Pset.get_element_pset(element, "EPset_Annotation")
+        if not pset:
+            if not classes:
+                return
+            pset = ifcopenshell.api.pset.add_pset(ifc_file, product=element, name="EPset_Annotation")
+        ifcopenshell.api.pset.edit_pset(ifc_file, pset=pset, properties={"Classes": " ".join(classes)})
+
+    @classmethod
+    def add_element_class(cls, element: ifcopenshell.entity_instance, name: str) -> None:
+        name = cls.sanitise_class_name(name)
+        if not name:
+            return
+        classes = cls.get_element_classes(element)
+        if name in classes:
+            return
+        cls.set_element_classes(element, classes + [name])
+
+    @classmethod
+    def remove_element_class(cls, element: ifcopenshell.entity_instance, name: str) -> None:
+        classes = cls.get_element_classes(element)
+        if name not in classes:
+            return
+        cls.set_element_classes(element, [c for c in classes if c != name])
+
+    @classmethod
+    def sanitise_class_name(cls, name: str) -> str:
+        """Strip characters that cannot appear in a CSS class name."""
+        name = re.sub(r"[^0-9a-zA-Z_-]+", "-", name.strip()).strip("-")
+        return re.sub(r"^[0-9-]+", "", name)
+
     @classmethod
     def get_annotation_z_index(cls, drawing: ifcopenshell.entity_instance) -> float:
         return ifcopenshell.util.element.get_pset(drawing, "EPset_Annotation", "ZIndex") or 0

diff --git a/src/bonsai/test/core/test_drawing.py b/src/bonsai/test/core/test_drawing.py
@@ -663,3 +663,15 @@ def test_create_a_missing_annotation_context_on_the_fly(self, ifc, collector, dr
             relating_type="element_type",
             enable_editing=True,
         )
+
+
+class TestAddElementClass:
+    def test_run(self, drawing):
+        drawing.add_element_class("element", "dashed").should_be_called()
+        subject.add_element_class(drawing, element="element", name="dashed")
+
+
+class TestRemoveElementClass:
+    def test_run(self, drawing):
+        drawing.remove_element_class("element", "dashed").should_be_called()
+        subject.remove_element_class(drawing, element="element", name="dashed")