diff --git a/fhircraft/fhir/resources/factory/builders/base.py b/fhircraft/fhir/resources/factory/builders/base.py
index e33a9cdd..723db947 100644
--- a/fhircraft/fhir/resources/factory/builders/base.py
+++ b/fhircraft/fhir/resources/factory/builders/base.py
@@ -442,6 +442,7 @@ def resolve_type(
     def build_primitive_extension_placeholder(
         self,
         node: ElementNode,
+        name: str | None = None,
     ) -> FieldInformation:
         """
         Build a placeholder field information for primitive type extensions.
@@ -450,14 +451,16 @@ def build_primitive_extension_placeholder(
 
         Args:
             node: ElementNode representing the primitive FHIR element for which to create an extension placeholder.
+            name: When provided, use this name instead of ``node.name``
 
         Returns:
             FieldInformation: Field information for the extension placeholder
         """
 
         # Process the name for the placeholder field
-        original_name = f"_{node.name}"
-        placeholder_name = f"{node.name}_ext"
+        name = name if name is not None else node.name
+        original_name = f"_{name}"
+        placeholder_name = f"{name}_ext"
         safe_placeholder_name, ext_alias = self.handle_python_keyword(placeholder_name)
 
         # Get the appropriate Element type for the placeholder field
@@ -470,7 +473,7 @@ def build_primitive_extension_placeholder(
             default=None,
             alias=original_name,
             validation_alias=ext_alias,
-            description=f"Placeholder element for {node.name} extensions",
+            description=f"Placeholder element for {name} extensions",
         )
         return info
 
diff --git a/fhircraft/fhir/resources/factory/builders/type_choice.py b/fhircraft/fhir/resources/factory/builders/type_choice.py
index 893e41fe..ed1f1347 100644
--- a/fhircraft/fhir/resources/factory/builders/type_choice.py
+++ b/fhircraft/fhir/resources/factory/builders/type_choice.py
@@ -54,7 +54,9 @@ def build(self, node: ElementNode, _: DefinitionIndex) -> Build:
 
             # Add primitive extension field if applicable
             if field_type_info.requires_primitive_extension:
-                placeholder = self.build_primitive_extension_placeholder(node)
+                placeholder = self.build_primitive_extension_placeholder(
+                    node, name=safe_name
+                )
                 build.fields.append(placeholder)
 
         # Type-choice validator & property go on the first FieldBuild
diff --git a/test/test_fhir_resources_factory_builders_type_choice.py b/test/test_fhir_resources_factory_builders_type_choice.py
index 6dd722aa..5a986115 100644
--- a/test/test_fhir_resources_factory_builders_type_choice.py
+++ b/test/test_fhir_resources_factory_builders_type_choice.py
@@ -214,7 +214,7 @@ def test_build__primitive_type_adds_ext_placeholder(builder, index):
     node = make_node("value", type_codes=["string"])
     result = builder.build(node, index)
     names = [f.name for f in result.fields]
-    assert "value_ext" in names
+    assert "valueString_ext" in names
 
 
 def test_build__non_primitive_type_does_not_add_placeholder(builder, index):
@@ -229,8 +229,51 @@ def test_build__non_primitive_type_does_not_add_placeholder(builder, index):
 def test_build__primitive_placeholder_alias_is_underscore_name(builder, index):
     node = make_node("value", type_codes=["string"])
     result = builder.build(node, index)
-    placeholder = next(f for f in result.fields if f.name == "value_ext")
-    assert placeholder.alias == "_value"
+    placeholder = next(f for f in result.fields if f.name == "valueString_ext")
+    assert placeholder.alias == "_valueString"
+
+
+def test_build__primitive_type_ext_placeholder_does_not_use_base_name(builder, index):
+    """No base-name _ext placeholder should be emitted for a type-choice element."""
+    node = make_node("value", type_codes=["string"])
+    result = builder.build(node, index)
+    names = [f.name for f in result.fields]
+    assert "value_ext" not in names
+
+
+def test_build__primitive_type_ext_placeholder_uses_typed_name(builder, index):
+    """Ext placeholder for a type-choice primitive must use the full typed field name."""
+    node = make_node("value", type_codes=["string"])
+    result = builder.build(node, index)
+    names = [f.name for f in result.fields]
+    assert "valueString_ext" in names
+
+
+def test_build__primitive_type_ext_placeholder_alias_uses_typed_name(builder, index):
+    """Ext placeholder alias must be the underscore-prefixed typed field name."""
+    node = make_node("value", type_codes=["string"])
+    result = builder.build(node, index)
+    placeholder = next(f for f in result.fields if f.name == "valueString_ext")
+    assert placeholder.alias == "_valueString"
+
+
+def test_build__two_primitives_ext_placeholders_use_typed_names(builder, index):
+    """Each primitive type choice variant gets its own correctly named _ext placeholder."""
+    node = make_node("value", type_codes=["string", "boolean"])
+    result = builder.build(node, index)
+    ext_names = [f.name for f in result.fields if f.name.endswith("_ext")]
+    assert "valueString_ext" in ext_names
+    assert "valueBoolean_ext" in ext_names
+    assert "value_ext" not in ext_names
+
+
+def test_build__different_base_name_primitive_ext_uses_typed_name(builder, index):
+    """The correctly-typed _ext placeholder is generated regardless of the base name."""
+    node = make_node("effective", type_codes=["dateTime"])
+    result = builder.build(node, index)
+    names = [f.name for f in result.fields]
+    assert "effectiveDateTime_ext" in names
+    assert "effective_ext" not in names
 
 
 def test_build__two_types_one_primitive_adds_exactly_one_placeholder(builder, index):