Planning Compliance from Above: Using Roof Data to Detect Unauthorised Extensions

Written by Stefani Mavrou on

Across England, the planning enforcement landscape has shifted. Since 25 April 2024, alleged operational development (including roof enlargements) can be enforced for up to 10 years if substantially completed on or after that date. In practice, councils have longer to act and a stronger incentive to triage suspected dormers and loft conversions efficiently, using remote evidence before committing officer time. GOV.UK+1

At the same time, new roof-level attribution in national mapping now describes roof shape, aspect and material, and flags solar and green roofs across tens of millions of buildings. When those attributes are aligned to authoritative building footprints, changes in roof geometry become measurable signals of recent works—legitimate or otherwise. Ordnance Survey+1

This post sets out a clean, technical workflow for using MapServe® roof layers plus OS MasterMap® base mapping to flag potential non-compliance from aerial data alone, anchored in current UK rules and recent developments.


The compliance questions roof data can actually answer

Most unauthorised dormers or loft build-outs fail on a short list of permitted development (PD) conditions that are quantifiable from geometry:

  • Volume of additional roof space:
    ≤40 m³ for a terraced house; ≤50 m³ for a semi-detached or detached house. Crucially, the cap is cumulative, so any previous enlargements count towards the same limit. Planning Portal

  • Fronting a highway:
    Roof extensions cannot project beyond the plane of the principal elevation that fronts a highway. If a dormer sits on that front slope, it’s not PD. ecab.planningportal.co.uk+1

  • Overall height:
    No part of the enlargement may exceed the highest point of the existing roof. A dormer that reaches or surpasses the main ridge breaches PD. Planning Portal

Those tests aren’t new, but two things are: (1) the 10-year enforcement window, which raises the stakes for early detection, and (2) richer roof attribution, which makes detection at scale practical. GOV.UK+1


What MapServe adds (and why that matters)

MapServe delivers CAD/GIS-ready OS MasterMap® as the authoritative building footprint baseline, with optional Building Heights, 1 m contours, and the new Roof layer (powered by OS NGD). In combination, these give you three things that general aerial tools don’t:

  1. Authoritative geometry: OS MasterMap® footprints are the national “source of truth” for building extents, updated regularly and used by the public sector. Align everything to this. Ordnance Survey

  2. Roof-level attribution: The MapServe roof layer exposes roof shape, aspect and material (and flags solar/green roofs) so you can isolate planes and detect form changes—hip-to-gable conversions, flat-roofed box dormers, etc. MapServe

  3. Heights for sanity checks: The Building Heights add-on provides indicative ridge/eaves figures to check that enlargements don’t overtop the original roof. Treat these as screening values and confirm on site for formal action. MapServe

Formats include DWG/DXF/GML/PDF/PNG, so you can work natively in AutoCAD, Revit, QGIS, ArcGIS or similar without time-consuming conversion. MapServe


A practical detection workflow (repeatable in CAD or GIS)

Below is a seven-step, rules-based pipeline that teams can run on single sites or batches. It uses plain geometry and attributes—no black-box AI—and is explainable to enforcement committees or the Planning Inspectorate.

1) Establish the baseline

  • Order OS MasterMap® for the subject property and immediate neighbours (context matters for PD).

  • Add MapServe Roof layer, Building Heights, and 1 m contours. Keep a consistent buffer (e.g., 50–100 m) to capture street context and garden depth. MapServe+2MapServe+2

2) Segment roof planes

  • Use roof attributes to split the roof into planes. Look for:

    • Loss of hip + new vertical gable plane → typical hip-to-gable conversion.

    • Flat/low-pitch insert on rear slope → likely box dormer.

    • Material contrast (e.g., GRP or single-ply patch within tiled field) → probable new element.

  • Record each suspect plane as its own polygon layer. Ordnance Survey

3) Compute areas and derived volume

  • For each suspected dormer plane, calculate plan area (m²).

  • Estimate added volume with a simple prism approximation:
    Volume ≈ plan area × dormer height (parapet/cheek height).

  • If available, use Building Heights to bound dormer height; store both the formula and inputs in the layer’s attributes for auditability. MapServe

4) Run the PD rule checks

  • Volume test: If the additional roof space exceeds 40 m³ (terrace) or 50 m³ (semi/detached)—or cumulative additions exceed that—mark Likely non-PD. Planning Portal

  • Front slope test: If a new plane intersects the principal elevation roof plane facing a highway, mark Consent required (not PD). ecab.planningportal.co.uk

  • Height test: If heights show any part at or above the original ridge, mark Likely non-PD; schedule a confirmatory site check. Planning Portal

5) Establish the “first-seen” date

  • Compile time-slice aerials (council captures, commercial orthos, historical imagery) and record the first date the dormer is visible. That date controls whether the 10-year enforcement window (post-25 April 2024) applies. Attach the image and date to your GIS feature for evidence. GOV.UK

6) Overlay constraints

  • Check Article 4 directions, conservation areas, and listing. If PD rights are removed or restricted, any roof enlargement likely needs permission regardless of geometry. (Retrieve the relevant LPA layers and store a boolean flag on the feature.)

  • Where roof attribution flags solar or green roofs, note these for design and materials conditions: they don’t legalise a dormer, but they do affect how a breach is rectified. Ordnance Survey

7) Output a triage pack

  • Generate a one-page sheet per property with:

    • Site plan (OS MasterMap® base) and highlighted suspect planes

    • Metrics (plan area, estimated volume, height deltas)

    • PD rule outcomes (Pass/Fail/Unclear)

    • First-seen date with thumbnail

    • Next step (e.g., “Owner letter”, “Site visit”, “Officer sign-off”)


Heuristics that catch most problems

You can score properties with simple, transparent rules to prioritise officer time:

  • Rear flat-roof plane ≥12 m² added to a pitched roof → +3 (probable dormer).

  • Hip removed, new gable plane on a previously hipped end → +2 (likely hip-to-gable).

  • Cheek/eaves set-back <~200 mm on plan → +2 (detail risk; common refusal reason).

  • Any intersection with front roof slope → +4 (not PD).

  • Height within 200 mm of ridge post-works → +3 (breach risk).

  • First-seen ≥25 Apr 2024 → +2 (inside the 10-year window). ecab.planningportal.co.uk+2Planning Portal+2

These aren’t legal determinations—just a rational way to push the highest-risk cases to the top of the queue.


Measurement notes (to keep you out of trouble)

  • Anchor to footprints: Use OS MasterMap® building outlines as your reference geometry. If a dormer has altered the footprint (e.g., oversailing into a rear outrigger), that’s a separate red flag; capture it as a delta layer. Ordnance Survey

  • Treat heights as indicative: MapServe’s Building Heights layer is excellent for screening, but it’s not a replacement for a measured survey where formal action is contemplated. Store the source of any height used for the decision trail. MapServe

  • Be disciplined with cumulative volume: Always check older imagery and approvals: the 40/50 m³ allowance is total across phases and previous owners. Planning Portal

  • Keep an audit trail: Save annotated screenshots and export layer attributes (CSV/GeoJSON). If a homeowner later seeks a certificate of lawfulness, your date evidence and calculations will matter. GOV.UK


Connecting the dots with current UK practice

A few 2025-specific developments make this approach even more relevant:

  • OS roof attribution went live nationally in March 2025, adding shape, aspect, material, solar and green roof presence for over 40 million buildings. These attributes are now being surfaced by data partners—including MapServe—so planning teams can analyse roof form changes systematically rather than eyeballing imagery. Ordnance Survey+1

  • Local authorities are scaling drone-enabled workflows (asset inspection, fly-over evidence capture), supported by the CAA’s continued regulatory updates in 2025. While you don’t need drones for desk-based screening, the regulatory environment is making aerial evidence more routine and defensible across public service operations. Research Reading+1

  • Enforcement horizons are clearer post-LURA: where works are substantially completed on or after 25 April 2024, the 10-year limit applies. That clarity helps LPAs justify proactive monitoring programmes and makes early anomaly detection from roof data proportionate and cost-effective. GOV.UK


Example: what a lightweight MapServe workflow looks like

  1. Download OS MasterMap® for the property plus a 100 m buffer in DWG (CAD) or GML (GIS). MapServe

  2. Overlay the Roof layer (shape/aspect/material; solar/green flags) and Building Heights. MapServe+1

  3. Digitise suspect planes; compute plan areas, estimate volume, and compare against 40/50 m³ thresholds. Planning Portal

  4. Check for any intersection with the front (principal elevation) slope facing a highway. If yes, mark not PD. ecab.planningportal.co.uk

  5. Attach a first-seen image and date from historical aerials; tag whether the 10-year window applies. GOV.UK

  6. Export a one-page sheet for the file and a CSV of metrics for batch reporting.

The result isn’t a legal verdict; it’s a defensible triage that lets enforcement teams spend their site hours where it matters, and helps advisers warn clients early when a design is likely to need permission.


Common pitfalls (and how to avoid them)

  • Assuming a front dormer can be PD: under householder rights it can’t if it projects beyond the principal elevation’s roof plane facing a highway; expect a refusal if you try to regularise it as PD. ecab.planningportal.co.uk

  • Over-relying on approximate heights: use indicative heights to screen; if you’re anywhere near the main ridge in your model, plan a measured check before you serve or advise. MapServe

  • Ignoring context designations: Article 4 directions and conservation areas can remove or restrict PD rights entirely; don’t assume a neat box dormer is lawful because the volumes work. (Confirm with LPA datasets.)

  • Forgetting cumulative volume: if a previous loft enlargement exists, the remaining allowance may be zero. Planning Portal


Why this matters to MapServe users

If you already source planning maps through MapServe, you have most of the building blocks:

  • Authoritative base mapping (OS MasterMap®) for clean measurement and presentation.

  • Roof-level attribution to spot form/material changes that betray dormers and hip-to-gable conversions.

  • Indicative heights to catch ridge breaches early.

  • Contours for context (e.g., checking whether apparent height changes are real or a datum illusion on sloping sites). MapServe

Tie those to simple PD rule checks and dated aerial evidence, and you’ve got a repeatable, auditable method to detect and prioritise suspected unauthorised roof extensions—fit for today’s 10-year enforcement reality. GOV.UK

Home