diff --git a/domain/data_transformation/__init__.py b/domain/data_transformation/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/infrastructure/column_classifier.py b/domain/data_transformation/column_classifier.py
similarity index 76%
rename from infrastructure/column_classifier.py
rename to domain/data_transformation/column_classifier.py
index b16fd8c0..adc88c6a 100644
--- a/infrastructure/column_classifier.py
+++ b/domain/data_transformation/column_classifier.py
@@ -16,14 +16,12 @@ class ClassificationError(Exception):
     """
 
 
-class BaseColumnClassifier(ABC, Generic[E]):
-    """Adapter base: shared scaffolding for concrete column classifiers.
+class ColumnClassifier(ABC, Generic[E]):
+    """Port: resolves free-text descriptions into a category enum ``E``.
 
     One classifier handles one landlord-CSV column. Implementations decide
-    *how* the mapping is performed (an LLM, a lookup table, a rules engine).
-    Consumers do not depend on this class -- they depend on the structural
-    ``ColumnClassifier`` Protocol declared in the orchestration layer; this
-    ABC merely gives adapters a common base for the ``classify`` contract.
+    *how* the mapping is performed (an LLM, a lookup table, a rules engine);
+    ``LandlordDescriptionOverridesOrchestrator`` depends only on this interface.
     """
 
     @abstractmethod
diff --git a/infrastructure/chatgpt/chatgpt_column_classifier.py b/infrastructure/chatgpt/chatgpt_column_classifier.py
index a42b9b4b..b782cf33 100644
--- a/infrastructure/chatgpt/chatgpt_column_classifier.py
+++ b/infrastructure/chatgpt/chatgpt_column_classifier.py
@@ -4,9 +4,9 @@ import json
 from enum import Enum
 from typing import Any, Optional, TypeVar
 
-from infrastructure.column_classifier import (
-    BaseColumnClassifier,
+from domain.data_transformation.column_classifier import (
     ClassificationError,
+    ColumnClassifier,
 )
 from infrastructure.chatgpt.chatgpt import ChatGPT
 from infrastructure.chatgpt.exceptions import ChatGPTClientError
@@ -14,7 +14,7 @@ from infrastructure.chatgpt.exceptions import ChatGPTClientError
 E = TypeVar("E", bound=Enum)
 
 
-class ChatGptColumnClassifier(BaseColumnClassifier[E]):
+class ChatGptColumnClassifier(ColumnClassifier[E]):
     """ColumnClassifier backed by ChatGPT, parametrised by a category enum.
 
     The same classification path -- prompt, JSON parsing, UNKNOWN fallback --
diff --git a/orchestration/classifiable_column.py b/orchestration/classifiable_column.py
index b8aeb08a..92334a38 100644
--- a/orchestration/classifiable_column.py
+++ b/orchestration/classifiable_column.py
@@ -2,8 +2,9 @@ from __future__ import annotations
 
 from dataclasses import dataclass
 from enum import Enum
-from typing import Generic, Protocol, TypeVar, runtime_checkable
+from typing import Generic, TypeVar
 
+from domain.data_transformation.column_classifier import ColumnClassifier
 from repositories.landlord_overrides.landlord_override_repository import (
     LandlordOverrideRepository,
 )
@@ -11,26 +12,6 @@ from repositories.landlord_overrides.landlord_override_repository import (
 E = TypeVar("E", bound=Enum)
 
 
-@runtime_checkable
-class ColumnClassifier(Protocol[E]):
-    """Port: resolves free-text descriptions into a category enum ``E``.
-
-    The orchestration layer owns this contract because it is the consumer.
-    Any object exposing a matching ``classify`` satisfies it structurally --
-    e.g. ``infrastructure.chatgpt.ChatGptColumnClassifier`` -- so orchestration
-    never imports an adapter.
-    """
-
-    def classify(self, descriptions: set[str]) -> dict[str, E]:
-        """Classify each description into a category enum member.
-
-        Every input description appears as a key in the result. A description
-        that cannot be resolved maps to the enum's UNKNOWN member. A wholesale
-        failure raises the adapter's ``ClassificationError``.
-        """
-        ...
-
-
 @dataclass(frozen=True)
 class ClassifiableColumn(Generic[E]):
     """Pairs a column's classifier with the repository that persists its results.
diff --git a/tests/infrastructure/chatgpt/test_chatgpt_column_classifier.py b/tests/infrastructure/chatgpt/test_chatgpt_column_classifier.py
index 0462f3ce..425d2625 100644
--- a/tests/infrastructure/chatgpt/test_chatgpt_column_classifier.py
+++ b/tests/infrastructure/chatgpt/test_chatgpt_column_classifier.py
@@ -4,7 +4,7 @@ from typing import Optional
 
 import pytest
 
-from infrastructure.column_classifier import ClassificationError
+from domain.data_transformation.column_classifier import ClassificationError
 from domain.epc.property_type import PropertyType
 from domain.epc.wall_type import WallType
 from infrastructure.chatgpt.chatgpt import ChatGPT
diff --git a/tests/orchestration/test_landlord_description_overrides_orchestrator.py b/tests/orchestration/test_landlord_description_overrides_orchestrator.py
index 1b9785c9..bf5b13ce 100644
--- a/tests/orchestration/test_landlord_description_overrides_orchestrator.py
+++ b/tests/orchestration/test_landlord_description_overrides_orchestrator.py
@@ -8,7 +8,8 @@ from domain.epc.built_form_type import BuiltFormType
 from domain.epc.property_type import PropertyType
 from domain.epc.wall_type import WallType
 from domain.postcode import Postcode
-from orchestration.classifiable_column import ClassifiableColumn, ColumnClassifier
+from domain.data_transformation.column_classifier import ColumnClassifier
+from orchestration.classifiable_column import ClassifiableColumn
 from orchestration.landlord_description_overrides_orchestrator import (
     LandlordDescriptionOverridesOrchestrator,
 )