diff --git a/backend/ecmk_fetcher/excel_writer.py b/backend/ecmk_fetcher/excel_writer.py
new file mode 100644
index 00000000..089e0187
--- /dev/null
+++ b/backend/ecmk_fetcher/excel_writer.py
@@ -0,0 +1,5 @@
+from typing import Any
+
+
+def write_row(file_path: str, row_data: dict[str, Any]) -> None:
+    raise NotImplementedError
diff --git a/backend/ecmk_fetcher/tests/test_excel_writer.py b/backend/ecmk_fetcher/tests/test_excel_writer.py
new file mode 100644
index 00000000..3f730951
--- /dev/null
+++ b/backend/ecmk_fetcher/tests/test_excel_writer.py
@@ -0,0 +1,123 @@
+import os
+import pathlib
+import pytest
+from openpyxl import load_workbook
+from openpyxl.worksheet.worksheet import Worksheet
+
+from backend.ecmk_fetcher.excel_writer import write_row
+
+
+@pytest.fixture
+def xlsx_path(tmp_path: pathlib.Path) -> str:
+    return str(tmp_path / "output.xlsx")
+
+
+def _active_sheet(file_path: str) -> Worksheet:
+    ws = load_workbook(file_path).active
+    assert isinstance(ws, Worksheet)
+    return ws
+
+
+def test_write_row_creates_file(xlsx_path: str):
+    # arrange
+    row = {
+        "address": "1 Fake Avenue, AB24 5CD",
+        "property_type": "House",
+        "main_dwelling_floor_1_area_m2": 43.61,
+    }
+
+    # act
+    write_row(xlsx_path, row)
+
+    # assert
+    assert os.path.exists(xlsx_path)
+    ws = _active_sheet(xlsx_path)
+    assert [c.value for c in ws[1]] == list(row.keys())
+    assert [c.value for c in ws[2]] == list(row.values())
+
+
+def test_write_row_appends_to_existing(xlsx_path: str):
+    # arrange
+    row_a = {
+        "address": "1 Fake Avenue, AB24 5CD",
+        "property_type": "House",
+        "main_dwelling_floor_1_area_m2": 43.61,
+    }
+    row_b = {
+        "address": "2 Other Street, XY1 2AB",
+        "property_type": "Flat",
+        "main_dwelling_floor_1_area_m2": 30.0,
+    }
+
+    # act
+    write_row(xlsx_path, row_a)
+    write_row(xlsx_path, row_b)
+
+    # assert
+    ws = _active_sheet(xlsx_path)
+    assert ws.max_row == 3  # 1 header + 2 data rows
+    assert [c.value for c in ws[1]] == list(row_a.keys())
+    assert [c.value for c in ws[2]] == list(row_a.values())
+    assert [c.value for c in ws[3]] == list(row_b.values())
+
+
+def test_write_row_inserts_new_columns_at_logical_positions(xlsx_path: str):
+    # arrange
+    # First row: main_dwelling floor 1 + roof
+    # Second row: also has main_dwelling floor 2 — should be inserted between floor 1 and roof,
+    # not appended to the end
+    row_a = {
+        "address": "1 Fake Avenue, AB24 5CD",
+        "property_type": "House",
+        "main_dwelling_floor_1_area_m2": 43.61,
+        "main_dwelling_floor_1_height_m": 2.46,
+        "main_dwelling_roof_construction": 4,
+    }
+    row_b = {
+        "address": "2 Other Street, XY1 2AB",
+        "property_type": "House",
+        "main_dwelling_floor_1_area_m2": 50.0,
+        "main_dwelling_floor_1_height_m": 2.5,
+        "main_dwelling_floor_2_area_m2": 48.0,
+        "main_dwelling_floor_2_height_m": 2.4,
+        "main_dwelling_roof_construction": 4,
+    }
+
+    # act
+    write_row(xlsx_path, row_a)
+    write_row(xlsx_path, row_b)
+
+    # assert
+    ws = _active_sheet(xlsx_path)
+
+    assert [c.value for c in ws[1]] == [
+        "address",
+        "property_type",
+        "main_dwelling_floor_1_area_m2",
+        "main_dwelling_floor_1_height_m",
+        "main_dwelling_floor_2_area_m2",  # inserted before roof, not at end
+        "main_dwelling_floor_2_height_m",
+        "main_dwelling_roof_construction",
+    ]
+
+    # row_a had no floor_2 data — those cells should be empty
+    assert [c.value for c in ws[2]] == [
+        "1 Fake Avenue, AB24 5CD",
+        "House",
+        43.61,
+        2.46,
+        None,  # main_dwelling_floor_2_area_m2
+        None,  # main_dwelling_floor_2_height_m
+        4,
+    ]
+
+    # row_b should be fully populated
+    assert [c.value for c in ws[3]] == [
+        "2 Other Street, XY1 2AB",
+        "House",
+        50.0,
+        2.5,
+        48.0,
+        2.4,
+        4,
+    ]