diff --git a/frappe/model/base_document.py b/frappe/model/base_document.py
index aa22772b9b..f02f9ac23e 100644
--- a/frappe/model/base_document.py
+++ b/frappe/model/base_document.py
@@ -127,7 +127,42 @@ def import_controller(doctype):
 	if not issubclass(class_, BaseDocument):
 		raise ImportError(f"{doctype}: {classname} is not a subclass of BaseDocument")
 
-	return class_
+	return get_extended_class(class_, doctype)
+
+
+def get_extended_class(base_class, doctype):
+	"""Create an extended class by mixing extension classes with the base class.
+
+	Args:
+		base_class: The base document class
+		doctype: The doctype name
+
+	Returns:
+		Extended class that combines all extension classes with the base class
+	"""
+
+	extensions = frappe.get_hooks("extend_doctype_class", {}).get(doctype)
+	if not extensions:
+		return base_class
+
+	# Get extension classes in reverse order using frappe.get_attr
+	extension_classes = []
+	for extension_path in reversed(extensions):
+		try:
+			extension_class = frappe.get_attr(extension_path)
+		except Exception:
+			frappe.throw(
+				_("Error retrieving extension class from path:<br><code>{0}</code>").format(extension_path)
+			)
+
+		extension_classes.append(extension_class)
+
+	# Create the extended class by combining extension classes with base class
+	# Extension classes come first in MRO, then base class
+	class_name = f"Extended{base_class.__name__}"
+	extended_class = type(class_name, (*extension_classes, base_class), {})
+
+	return extended_class
 
 
 RESERVED_KEYWORDS = frozenset(
diff --git a/frappe/tests/test_base_document.py b/frappe/tests/test_base_document.py
index 59f0dcf81b..e08e9def5b 100644
--- a/frappe/tests/test_base_document.py
+++ b/frappe/tests/test_base_document.py
@@ -1,7 +1,30 @@
-from frappe.model.base_document import BaseDocument
+import frappe
+from frappe.desk.doctype.todo.todo import ToDo
+from frappe.model.base_document import BaseDocument, get_extended_class
 from frappe.tests import IntegrationTestCase
 
 
+class TestExtensionA(BaseDocument):
+	def extension_method_a(self):
+		return "method_a"
+
+
+class TestExtensionB(BaseDocument):
+	def extension_method_b(self):
+		return "method_b"
+
+
+class TestToDoExtension(BaseDocument):
+	"""Extension class that overrides ToDo's validate method"""
+
+	def validate(self):
+		# Add our custom logic
+		self.custom_validation_called = True
+
+	def extension_method(self):
+		return "extension_method_called"
+
+
 class TestBaseDocument(IntegrationTestCase):
 	def test_docstatus(self):
 		doc = BaseDocument({"docstatus": 0, "doctype": "ToDo"})
@@ -15,3 +38,106 @@ class TestBaseDocument(IntegrationTestCase):
 		doc.docstatus = 2
 		self.assertTrue(doc.docstatus.is_cancelled())
 		self.assertEqual(doc.docstatus, 2)
+
+	def test_get_extended_class_with_no_extensions(self):
+		"""Test that get_extended_class returns the base class when no extensions are provided."""
+
+		with self.patch_hooks({"extend_doctype_class": {}}):
+			result = get_extended_class(ToDo, "ToDo")
+			self.assertEqual(result, ToDo)
+
+		with self.patch_hooks({"extend_doctype_class": {"ToDo": []}}):
+			result = get_extended_class(ToDo, "ToDo")
+			self.assertEqual(result, ToDo)
+
+	def test_get_extended_class_with_extensions(self):
+		"""Test that get_extended_class properly combines extension classes with base class."""
+		# Mock frappe.get_hooks to return extension paths
+		extensions = [
+			"frappe.tests.test_base_document.TestExtensionA",
+			"frappe.tests.test_base_document.TestExtensionB",
+		]
+
+		with self.patch_hooks({"extend_doctype_class": {"ToDo": extensions}}):
+			extended_class = get_extended_class(ToDo, "ToDo")
+
+			# Test that the extended class is different from base class
+			self.assertNotEqual(extended_class, ToDo)
+
+			# Test that the extended class has all methods from extensions and base
+			instance = extended_class({"doctype": "ToDo"})
+			self.assertTrue(hasattr(instance, "extension_method_a"))
+			self.assertTrue(hasattr(instance, "extension_method_b"))
+
+			# Test that methods work correctly
+			self.assertEqual(instance.extension_method_a(), "method_a")
+			self.assertEqual(instance.extension_method_b(), "method_b")
+
+			# Test MRO (Method Resolution Order) - extensions should come first in reverse order
+			mro_classes = [cls.__name__ for cls in extended_class.__mro__]
+			self.assertIn("TestExtensionB", mro_classes)
+			self.assertIn("TestExtensionA", mro_classes)
+			self.assertIn("ToDo", mro_classes)
+
+			# TestExtensionB should come before TestExtensionA (reverse order)
+			idx_b = mro_classes.index("TestExtensionB")
+			idx_a = mro_classes.index("TestExtensionA")
+			idx_base = mro_classes.index("ToDo")
+			self.assertLess(idx_b, idx_a)
+			self.assertLess(idx_a, idx_base)
+
+	def test_extension_overrides_todo_method(self):
+		"""Test that an extension can override methods from the actual ToDo class"""
+		from frappe.desk.doctype.todo.todo import ToDo
+
+		# Mock the hooks to include our ToDo extension
+		extensions = ["frappe.tests.test_base_document.TestToDoExtension"]
+
+		with self.patch_hooks({"extend_doctype_class": {"ToDo": extensions}}):
+			extended_class = get_extended_class(ToDo, "ToDo")
+
+			# Test that the extended class is different from base ToDo
+			self.assertNotEqual(extended_class, ToDo)
+
+			# Create an instance of the extended ToDo
+			instance = extended_class({"doctype": "ToDo"})
+
+			# Test that extension method is available
+			self.assertTrue(hasattr(instance, "extension_method"))
+			self.assertEqual(instance.extension_method(), "extension_method_called")
+
+			# Test that the validate method is overridden
+			# The extension's validate method should set custom_validation_called = True
+			instance.validate()
+			self.assertTrue(getattr(instance, "custom_validation_called", False))
+
+			# Test MRO - extension should come before ToDo class
+			mro_classes = [cls.__name__ for cls in extended_class.__mro__]
+			self.assertIn("TestToDoExtension", mro_classes)
+			self.assertIn("ToDo", mro_classes)
+
+			# TestToDoExtension should come before ToDo
+			idx_extension = mro_classes.index("TestToDoExtension")
+			idx_todo = mro_classes.index("ToDo")
+			self.assertLess(idx_extension, idx_todo)
+
+	def test_extension_invalid_path_raises_exception(self):
+		"""Test that an invalid extension path raises an appropriate exception"""
+		from frappe.desk.doctype.todo.todo import ToDo
+
+		# Mock the hooks to include an invalid extension path
+		path_to_invalid_extension = "invalid.module.path.NonExistentClass"
+
+		extensions = [
+			"frappe.tests.test_base_document.TestExtensionA",  # valid
+			path_to_invalid_extension,  # invalid
+		]
+
+		with self.patch_hooks({"extend_doctype_class": {"ToDo": extensions}}):
+			# Test that frappe.ValidationError is raised for invalid extension path
+			with self.assertRaises(frappe.ValidationError) as context:
+				get_extended_class(ToDo, "ToDo")
+
+			# Check that the error message mentions the invalid path
+			error_message = str(context.exception)
+			self.assertIn(path_to_invalid_extension, error_message)