diff --git a/src/bonsai/bonsai/bim/module/cost/ui.py b/src/bonsai/bonsai/bim/module/cost/ui.py
index 738e793fba..c1a8e3a1ae 100644
--- a/src/bonsai/bonsai/bim/module/cost/ui.py
+++ b/src/bonsai/bonsai/bim/module/cost/ui.py
@@ -38,6 +38,8 @@ class BIM_PT_cost_schedules(Panel):
     bl_parent_id = "BIM_PT_tab_cost"
     bl_options = {"HIDE_HEADER"}
 
+    layout: bpy.types.UILayout
+
     @classmethod
     def poll(cls, context):
         file = tool.Ifc.get()
@@ -138,13 +140,13 @@ class BIM_PT_cost_schedules(Panel):
                     self.draw_currency_ui()
                 self.draw_editable_cost_item_ui()
 
-    def draw_column_ui(self):
+    def draw_column_ui(self) -> None:
         row = self.layout.row(align=True)
         row.prop(self.props, "cost_column", text="")
         row.operator("bim.add_cost_column", text="", icon="ADD").name = self.props.cost_column
         self.layout.template_list("BIM_UL_cost_columns", "", self.props, "columns", self.props, "active_column_index")
 
-    def draw_currency_ui(self):
+    def draw_currency_ui(self) -> None:
         row = self.layout.row(align=True)
         if CostSchedulesData.data["currency"]:
             text = "Currency used: {}".format(CostSchedulesData.data["currency"]["name"])
@@ -161,7 +163,7 @@ class BIM_PT_cost_schedules(Panel):
     def draw_editable_cost_schedule_ui(self):
         bonsai.bim.helper.draw_attributes(self.props.cost_schedule_attributes, self.layout)
 
-    def draw_editable_cost_item_ui(self):
+    def draw_editable_cost_item_ui(self) -> None:
         row = self.layout.row(align=True)
         row.alignment = "RIGHT"
         ifc_definition_id = None
@@ -217,7 +219,7 @@ class BIM_PT_cost_schedules(Panel):
             elif self.props.cost_item_editing_type == "VALUES":
                 self.draw_editable_cost_item_values_ui()
 
-    def draw_editable_cost_item_attributes_ui(self):
+    def draw_editable_cost_item_attributes_ui(self) -> None:
         bonsai.bim.helper.draw_attributes(self.props.cost_item_attributes, self.layout)
 
     def draw_editable_cost_item_quantities_ui(self, cost_item: dict[str, Any]):
diff --git a/src/ifcopenshell-python/ifcopenshell/api/cost/add_cost_item.py b/src/ifcopenshell-python/ifcopenshell/api/cost/add_cost_item.py
index c03fadbe64..34e20aa63a 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/cost/add_cost_item.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/cost/add_cost_item.py
@@ -56,20 +56,18 @@ def add_cost_item(
         # Alternatively you may add them as subitems
         item2 = ifcopenshell.api.cost.add_cost_item(model, cost_item=item1)
     """
-    settings = {"cost_schedule": cost_schedule, "cost_item": cost_item}
+    cost_item_ = ifcopenshell.api.root.create_entity(file, ifc_class="IfcCostItem")
 
-    cost_item = ifcopenshell.api.root.create_entity(file, ifc_class="IfcCostItem")
-
-    if settings["cost_schedule"]:
+    if cost_schedule:
         file.create_entity(
             "IfcRelAssignsToControl",
             **{
                 "GlobalId": ifcopenshell.guid.new(),
                 "OwnerHistory": ifcopenshell.api.owner.create_owner_history(file),
-                "RelatedObjects": [cost_item],
-                "RelatingControl": settings["cost_schedule"],
+                "RelatedObjects": [cost_item_],
+                "RelatingControl": cost_schedule,
             },
         )
-    elif settings["cost_item"]:
-        ifcopenshell.api.nest.assign_object(file, related_objects=[cost_item], relating_object=settings["cost_item"])
-    return cost_item
+    elif cost_item:
+        ifcopenshell.api.nest.assign_object(file, related_objects=[cost_item_], relating_object=cost_item)
+    return cost_item_
diff --git a/src/ifcopenshell-python/ifcopenshell/ifcopenshell_wrapper.pyi b/src/ifcopenshell-python/ifcopenshell/ifcopenshell_wrapper.pyi
index 17c2d14a36..2c0a687397 100644
--- a/src/ifcopenshell-python/ifcopenshell/ifcopenshell_wrapper.pyi
+++ b/src/ifcopenshell-python/ifcopenshell/ifcopenshell_wrapper.pyi
@@ -17,7 +17,7 @@
 # along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
 
 import ifcopenshell
-from typing import Any, Union, Literal
+from typing import Any, Union, Literal, Self
 
 # `std::vector<xxx>` usually translated to `tuple[xxx, ...]`.
 
@@ -731,14 +731,17 @@ class collection:
     def matrix(self): ...
 
 class colour(item):
-    def b(self): ...
-    def calc_hash(self): ...
-    def clone_(self): ...
+    def r(self) -> float: ...
+    def g(self) -> float: ...
+    def b(self) -> float: ...
+    def calc_hash(self) -> int: ...
+    def clone_(self) -> Self: ...
     @property
-    def components(self): ...
-    def g(self): ...
-    def kind(self): ...
-    def r(self): ...
+    def components(self) -> tuple[float, float, float]:
+        """RGB tuple."""
+        ...
+
+    def kind(self) -> int: ...
 
 class context:
     def add(self, segments): ...
@@ -1087,11 +1090,14 @@ class item:
     instance: Any
     orientation: Any
     def calc_hash(self): ...
-    def clone_(self): ...
+    # TODO: hide from the wrapper?
+    def clone_(self) -> Self: ...
     def hash(self): ...
     def identity(self): ...
-    def kind(self): ...
     def reverse(self): ...
+    # TODO: hide from the wrapper?
+    def kind(self):
+        """Internal enum item index."""
 
 class less_functor: ...
 
@@ -1329,18 +1335,25 @@ class sphere(surface):
     def matrix(self): ...
 
 class style(item):
-    diffuse: Any
-    name: Any
-    specular: Any
-    specularity: Any
-    surface: Any
-    transparency: Any
-    use_surface_color: Any
-    def calc_hash(self): ...
-    def clone_(self): ...
-    def get_color(self): ...
-    def has_specularity(self): ...
-    def has_transparency(self): ...
+    diffuse: colour
+    name: str
+    """E.g. 'IfcSurfaceStyleShading-218', where 218 is style's STEP id."""
+    specular: colour
+    specularity: float
+    """Can be `float('nan')`"""
+    surface: colour
+    transparency: float
+    """Can be `float('nan')`"""
+    use_surface_color: bool
+
+    def calc_hash(self) -> int: ...
+    def clone_(self) -> Self: ...
+    def get_color(self) -> colour:
+        """ "Return surface color if it's used, otherwise return diffuse color."""
+        ...
+
+    def has_specularity(self) -> bool: ...
+    def has_transparency(self) -> bool: ...
     def instance_id(self) -> int:
         """
         Possible values for `instance_id`:
@@ -1354,7 +1367,7 @@ class style(item):
         """
         ...
 
-    def kind(self): ...
+    def kind(self) -> int: ...
 
 class surface(geom_item): ...