diff --git a/STACpopulator/extensions/base.py b/STACpopulator/extensions/base.py
index 56082a1..ca74821 100644
--- a/STACpopulator/extensions/base.py
+++ b/STACpopulator/extensions/base.py
@@ -22,6 +22,7 @@
 
 
 """
+from __future__ import annotations
 
 from datetime import datetime
 import json
@@ -54,7 +55,7 @@
 LOGGER = logging.getLogger(__name__)
 
 
-class DataModelHelper(BaseModel):
+class ExtensionHelper(BaseModel):
     """Base class for dataset properties going into the catalog.
 
     Subclass this with attributes.
@@ -103,7 +104,51 @@ def apply(self, item, add_if_missing=False):
         return item
 
 
-class THREDDSCatalogDataModel(BaseModel):
+class BaseSTAC(BaseModel):
+    """Base class for STAC item data models."""
+    # STAC item properties
+    geometry: AnyGeometry | None
+    bbox: list[float]
+    start_datetime: datetime
+    end_datetime: datetime
+
+    extensions: list = []
+
+    model_config = ConfigDict(populate_by_name=True, extra="ignore", arbitrary_types_allowed=True)
+
+    @property
+    def uid(self) -> str:
+        """Return a unique ID. When subclassing, use a combination of properties uniquely identifying a dataset."""
+        # TODO: Should this be an abstract method?
+        import uuid
+        return str(uuid.uuid4())
+
+    def stac_item(self) -> "pystac.Item":
+        """Create a STAC item and add extensions."""
+        item = pystac.Item(
+            id=self.uid,
+            geometry=self.geometry.model_dump(),
+            bbox=self.bbox,
+            properties={
+                "start_datetime": str(self.start_datetime),
+                "end_datetime": str(self.end_datetime),
+            },
+            datetime=None,
+        )
+
+        # Add extensions
+        for ext in self.extensions:
+            getattr(self, ext).apply(item)
+
+        try:
+            item.validate()
+        except STACValidationError as e:
+            raise Exception("Failed to validate STAC item") from e
+
+        return json.loads(json.dumps(item.to_dict()))
+
+
+class THREDDSCatalogDataModel(BaseSTAC):
     """Base class ingesting attributes loaded by `THREDDSLoader` and creating a STAC item.
 
     This is meant to be subclassed for each extension.
@@ -112,18 +157,11 @@ class THREDDSCatalogDataModel(BaseModel):
      - pydantic validation using type hints, and
      - json schema validation.
     """
-
-    # STAC item properties
-    geometry: GeoJSONPolygon
-    bbox: list[float]
-    start_datetime: datetime
-    end_datetime: datetime
-
     # Data from loader
     data: dict
 
     # Extensions classes
-    properties: DataModelHelper
+    properties: ExtensionHelper
     datacube: DataCubeHelper
     thredds: THREDDSHelper
 
@@ -165,37 +203,6 @@ def thredds_helper(cls, data):
         data["thredds"] = THREDDSHelper(data['data']["access_urls"])
         return data
 
-    @property
-    def uid(self) -> str:
-        """Return a unique ID. When subclassing, use a combination of properties uniquely identifying a dataset."""
-        # TODO: Should this be an abstract method?
-        import uuid
-        return str(uuid.uuid4())
-
-    def stac_item(self) -> "pystac.Item":
-        """Create a STAC item and add extensions."""
-        item = pystac.Item(
-            id=self.uid,
-            geometry=self.geometry.model_dump(),
-            bbox=self.bbox,
-            properties={
-                "start_datetime": str(self.start_datetime),
-                "end_datetime": str(self.end_datetime),
-            },
-            datetime=None,
-        )
-
-        # Add extensions
-        for ext in self.extensions:
-            getattr(self, ext).apply(item)
-
-        try:
-            item.validate()
-        except STACValidationError as e:
-            raise Exception("Failed to validate STAC item") from e
-
-        return json.loads(json.dumps(item.to_dict()))
-
 
 def metacls_extension(name, schema_uri):
     """Create an extension class dynamically from the properties."""
diff --git a/STACpopulator/extensions/cordex6.py b/STACpopulator/extensions/cordex6.py
index 12eed34..e16e14b 100644
--- a/STACpopulator/extensions/cordex6.py
+++ b/STACpopulator/extensions/cordex6.py
@@ -7,11 +7,11 @@
 from importlib import reload
 import STACpopulator.extensions.base
 reload(STACpopulator.extensions.base)
-from STACpopulator.extensions.base import THREDDSCatalogDataModel, DataModelHelper
+from STACpopulator.extensions.base import THREDDSCatalogDataModel, ExtensionHelper
 
 
 # This is generated using datamodel-codegen + manual edits
-class CordexCmip6(DataModelHelper):
+class CordexCmip6(ExtensionHelper):
     # Fields from schema
     activity_id: str = Field(..., alias='cordex6:activity_id')
     contact: str = Field(..., alias='cordex6:contact')