diff --git a/backend/magic_plan/audit_script.py b/backend/magic_plan/audit_script.py
index 202aa8ac..39ad2d44 100644
--- a/backend/magic_plan/audit_script.py
+++ b/backend/magic_plan/audit_script.py
@@ -2,7 +2,7 @@ from dataclasses import dataclass
 from typing import Optional
 
 import openpyxl
-from openpyxl.styles import Alignment, Font, PatternFill
+from openpyxl.styles import Alignment, Border, Font, PatternFill, Side
 from openpyxl.worksheet.worksheet import Worksheet
 
 from datatypes.magicplan.domain.models import Plan
@@ -85,6 +85,15 @@ def _build_rows(plan: Plan) -> list[AuditRow]:
 _BLANK_SEPARATOR_COLS = {5, 21}
 
 
+_MEDIUM_RIGHT = Border(right=Side(style="medium"))
+_SECTION_BOUNDARY_COLS = {4, 11, 15, 20}
+
+
+def _apply_section_borders(ws: Worksheet, row: int) -> None:
+    for col in _SECTION_BOUNDARY_COLS:
+        ws.cell(row=row, column=col).border = _MEDIUM_RIGHT
+
+
 def _write_headers(ws: Worksheet) -> None:
     # Row 1: group labels (merged)
     _merge_bold_fill(ws, 1, 1, 1, 4, "Room")
diff --git a/backend/magic_plan/tests/test_audit_script.py b/backend/magic_plan/tests/test_audit_script.py
index 8e966561..e7160eb7 100644
--- a/backend/magic_plan/tests/test_audit_script.py
+++ b/backend/magic_plan/tests/test_audit_script.py
@@ -1,6 +1,6 @@
 import openpyxl
 from datatypes.magicplan.domain.models import Door, Floor, Plan, Room, Window
-from backend.magic_plan.audit_script import AuditRow, _build_rows, _write_headers
+from backend.magic_plan.audit_script import AuditRow, _apply_section_borders, _build_rows, _write_headers
 
 
 def test_build_rows_opening_and_trickle_vent_fields_are_none() -> None:
@@ -260,3 +260,19 @@ def test_write_headers_two_rows_correct_labels_and_column_positions() -> None:
     assert ws.cell(row=2, column=13).value == "Opening Width m"
     assert ws.cell(row=2, column=14).value == "Opening Height m"
     assert ws.cell(row=2, column=15).value == "Total Area m²"
+
+
+def test_apply_section_borders_sets_medium_right_border_on_boundary_columns() -> None:
+    # Arrange
+    wb = openpyxl.Workbook()
+    ws = wb.active  # type: ignore[assignment]
+
+    # Act
+    _apply_section_borders(ws, 1)
+
+    # Assert — medium right-border on last col of each subtable
+    for col in (4, 11, 15, 20):
+        assert ws.cell(row=1, column=col).border.right.style == "medium", f"col {col}"
+    # Assert — non-boundary columns are untouched
+    for col in (1, 6, 12, 22):
+        assert ws.cell(row=1, column=col).border.right.style is None, f"col {col}"