diff --git a/src/bo4e/bo/geschaeftsobjekt.py b/src/bo4e/bo/geschaeftsobjekt.py
index 0c33cdd7e..3607809fc 100644
--- a/src/bo4e/bo/geschaeftsobjekt.py
+++ b/src/bo4e/bo/geschaeftsobjekt.py
@@ -35,7 +35,7 @@ class Geschaeftsobjekt(BaseModel):
     # optional attributes
     externe_referenzen: Optional[List[ExterneReferenz]] = []
     #: Hier können IDs anderer Systeme hinterlegt werden (z.B. eine SAP-GP-Nummer oder eine GUID)
-
+    # pylint:disable=duplicate-code
     class Config:
         """
         basic configuration for pydantic's behaviour
@@ -43,4 +43,5 @@ class Config:
 
         alias_generator = camelize
         allow_population_by_field_name = True
+        extra = "allow"
         json_encoders = {Decimal: str}
diff --git a/src/bo4e/com/com.py b/src/bo4e/com/com.py
index 384e8f7c4..52c086ad8 100644
--- a/src/bo4e/com/com.py
+++ b/src/bo4e/com/com.py
@@ -26,6 +26,7 @@ class COM(BaseModel):
 
     """
 
+    # pylint:disable=duplicate-code
     class Config:
         """
         basic configuration for pydantic's behaviour
@@ -33,10 +34,13 @@ class Config:
 
         alias_generator = camelize
         allow_population_by_field_name = True
+        extra = "allow"
         json_encoders = {Decimal: str}
 
 
 # pylint: disable=invalid-name
 #: Any type derived from COM including those that do not directly inherit from COM
 TCom = TypeVar("TCom", bound=Type[COM])
+
+
 # todo: find out if this way of typing is correct
diff --git a/tests/test_externe_referenz.py b/tests/test_externe_referenz.py
index 19cf0e991..16bec8fac 100644
--- a/tests/test_externe_referenz.py
+++ b/tests/test_externe_referenz.py
@@ -1,3 +1,5 @@
+from typing import Any, Dict
+
 from bo4e.bo.geschaeftspartner import Geschaeftspartner
 from bo4e.com.adresse import Adresse
 from bo4e.com.externereferenz import ExterneReferenz
@@ -61,3 +63,14 @@ def test_geschaeftspartner_with_no_externe_referenz(self) -> None:
         deserialized_gp: Geschaeftspartner = Geschaeftspartner.parse_raw(gp_json)
 
         assert deserialized_gp.externe_referenzen == []
+
+    def test_extension_data(self) -> None:
+        """
+        tests the behaviour of the json extension data (`extra="allow"`)
+        """
+        er = ExterneReferenz(ex_ref_name="foo.bar", ex_ref_wert="12345")
+        er_json: Dict[str, Any] = er.dict()
+        er_json["additional_key"] = "additional_value"
+        deserialized_er: ExterneReferenz = ExterneReferenz.parse_obj(er_json)
+        assert isinstance(deserialized_er, ExterneReferenz)
+        assert deserialized_er.additional_key == "additional_value"  # type:ignore[attr-defined]
diff --git a/tests/test_messlokation.py b/tests/test_messlokation.py
index d7ff3d6f8..63f5e2985 100644
--- a/tests/test_messlokation.py
+++ b/tests/test_messlokation.py
@@ -1,6 +1,7 @@
 import json
 from datetime import datetime
 from decimal import Decimal
+from typing import Any, Dict
 
 import pytest
 from pydantic import ValidationError
@@ -214,3 +215,17 @@ def _instantiate_melo(melo_id: str) -> None:
                 _instantiate_melo(melo_id)
         else:
             _instantiate_melo(melo_id)
+
+    def test_extension_data(self) -> None:
+        """
+        tests the behaviour of the json extension data (`extra="allow"`)
+        """
+        melo = Messlokation(
+            messlokations_id="DE00056266802AO6G56M11SN51G21M24S",
+            sparte=Sparte.STROM,
+        )
+        melo_json: Dict[str, Any] = melo.dict()
+        melo_json["additional_key"] = "additional_value"
+        deserialized_melo: Messlokation = Messlokation.parse_obj(melo_json)
+        assert isinstance(deserialized_melo, Messlokation)
+        assert deserialized_melo.additional_key == "additional_value"  # type:ignore[attr-defined]