You can now add cost items into a cost schedule, and as sub items to a parent cost item. Thanks myoualid and Jesusbill!

src/ifcopenshell-python/ifcopenshell/api/cost/add_cost_item.py

@@ -0,0 +1,28 @@
+import ifcopenshell.api
+
+
+class Usecase:
+    def __init__(self, file, **settings):
+        self.file = file
+        self.settings = {"cost_schedule": None, "cost_item": None}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        cost_item = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcCostItem")
+
+        if self.settings["cost_schedule"]:
+            self.file.create_entity(
+                "IfcRelAssignsToControl",
+                **{
+                    "GlobalId": ifcopenshell.guid.new(),
+                    "OwnerHistory": ifcopenshell.api.run("owner.create_owner_history", self.file),
+                    "RelatedObjects": [cost_item],
+                    "RelatingControl": self.settings["cost_schedule"],
+                }
+            )
+        elif self.settings["cost_item"]:
+            ifcopenshell.api.run(
+                "nest.assign_object", self.file, object=cost_item, relating_object=self.settings["cost_item"]
+            )
+        return cost_item