diff --git a/harness/report.py b/harness/report.py
index cfa8d570..df36a05c 100644
--- a/harness/report.py
+++ b/harness/report.py
@@ -330,3 +330,65 @@ def format_report_markdown(reports: list[PropertyReport]) -> str:
         *_measures_triggers_section(reports),
     ]
     return "\n".join(sections).rstrip() + "\n"
+
+
+_CSV_HEADER: Final[str] = (
+    "cert,lodged_sap,calculated_sap,sap_error,sap_error_flag,"
+    "baseline_sap,post_sap,baseline_band,post_band,measures,measure_types,"
+    "cost_of_works,contingency,bill_savings,co2_savings,valuation_pct,"
+    "triggers,error"
+)
+
+
+def _csv_cell(value: object) -> str:
+    """Render a CSV cell, rounding floats and keeping the row comma-safe
+    (commas in any value become ';' so the column count never changes)."""
+    if value is None:
+        return ""
+    if isinstance(value, float):
+        return f"{value:.2f}"
+    return str(value).replace(",", ";")
+
+
+def _csv_triggers(report: PropertyReport) -> str:
+    """Flatten the fired measures and their triggers into one comma-safe cell:
+    `type(field=value;field=value)|type(field=value)`."""
+    return "|".join(
+        f"{trigger.measure_type}("
+        + ";".join(f"{field}={value}" for field, value in trigger.triggers.items())
+        + ")"
+        for trigger in report.measure_triggers
+    )
+
+
+def format_report_csv(reports: list[PropertyReport]) -> str:
+    """Render the report as a flat CSV — one row per property, browsable and
+    sortable in a spreadsheet for a large dump. The calculator-error fields, the
+    Plan headline figures, and the flattened triggers all share one row."""
+    rows: list[str] = [_CSV_HEADER]
+    for report in reports:
+        plan: Optional[Plan] = report.plan
+        cells: list[object] = [
+            report.name,
+            report.lodged_sap,
+            report.calculated_sap,
+            report.sap_error,
+            1 if report.sap_error_exceeds_threshold else 0,
+            None if plan is None else plan.baseline.sap_continuous,
+            None if plan is None else plan.post_sap_continuous,
+            None if plan is None else plan.baseline_epc_rating.value,
+            None if plan is None else plan.post_epc_rating.value,
+            None if plan is None else len(plan.measures),
+            None
+            if plan is None
+            else ";".join(measure.measure_type for measure in plan.measures),
+            None if plan is None else plan.cost_of_works,
+            None if plan is None else plan.contingency_cost,
+            None if plan is None else plan.energy_bill_savings,
+            None if plan is None else plan.co2_savings_kg_per_yr,
+            None if plan is None else plan.valuation.average_pct * 100,
+            _csv_triggers(report),
+            report.calculator_error or report.plan_error,
+        ]
+        rows.append(",".join(_csv_cell(cell) for cell in cells))
+    return "\n".join(rows)
diff --git a/tests/harness/test_report.py b/tests/harness/test_report.py
index 9f84c905..0e54350f 100644
--- a/tests/harness/test_report.py
+++ b/tests/harness/test_report.py
@@ -11,6 +11,7 @@ from harness.report import (
     PropertyReport,
     build_property_report,
     build_property_reports,
+    format_report_csv,
     format_report_markdown,
     parity_report_for,
 )
@@ -188,6 +189,40 @@ def test_markdown_renders_the_three_sections(tmp_path: Path) -> None:
     assert "floor_construction_type" in markdown
 
 
+def test_csv_has_one_row_per_property_with_flags_and_triggers(tmp_path: Path) -> None:
+    # Arrange
+    bad: Path = tmp_path / "broken.json"
+    bad.write_text(json.dumps({"not": "an epc"}))
+    reports: list[PropertyReport] = build_property_reports(
+        [
+            _GOLDEN / f"{_WITHIN_TOLERANCE}.json",
+            _GOLDEN / f"{_DIVERGENT}.json",
+            bad,
+        ]
+    )
+
+    # Act
+    csv: str = format_report_csv(reports)
+
+    # Assert — header plus one row per property.
+    lines: list[str] = csv.splitlines()
+    assert lines[0].startswith("cert,")
+    assert len(lines) == len(reports) + 1
+    # Every data row is comma-safe (no row splits into extra columns).
+    column_count: int = len(lines[0].split(","))
+    assert all(len(line.split(",")) == column_count for line in lines[1:])
+    rows: dict[str, str] = {line.split(",")[0]: line for line in lines[1:]}
+    # The divergent cert carries the |Δ| > 0.5 flag and the within-tolerance one doesn't.
+    flag_index: int = lines[0].split(",").index("sap_error_flag")
+    assert rows[_DIVERGENT].split(",")[flag_index] == "1"
+    assert rows[_WITHIN_TOLERANCE].split(",")[flag_index] == "0"
+    # The measure-bearing cert flattens its triggers into the row.
+    assert "solid_floor_insulation" in rows[_WITHIN_TOLERANCE]
+    assert "floor_construction_type=Solid" in rows[_WITHIN_TOLERANCE]
+    # The unscorable cert keeps its error.
+    assert "ValueError" in rows["broken"]
+
+
 def test_unparseable_cert_is_captured_not_raised(tmp_path: Path) -> None:
     # Arrange — a payload the mapper rejects must not abort the report.
     bad: Path = tmp_path / "broken.json"