Rear extension in Harrow
Rear extension in London — permitted-development limits, costs and the London-wide route. This page is the Harrow position.
Do you need planning permission for a rear extension in Harrow?
Outside the conservation areas, much of Harrow's interwar suburban stock keeps permitted development rights for a single-storey rear extension within the limits, or larger via prior approval, to the Residential Design Guide SPD. Inside a conservation area with householder Article 4 control, even minor external alterations need a full application. Check the address first.
Interwar semis in Pinner, Stanmore and Kenton extend routinely under PD; the Harrow on the Hill and Pinner conservation areas need a full application.
Harrow contrasts a historic hilltop core with Metro-land suburbs of generous interwar semis and Edwardian streets. Outside the protected clusters, rear extensions are staple projects on wide plots with side access. The Residential Design Guide SPD — incorporating the council's householder extensions guide — sets scale, materials, daylight and neighbour expectations for applications that do need permission.
What actually applies in Harrow
Conservation areas in Harrow
Real · planning.data.gov.ukEvery designated conservation area in Harrow from the official dataset — inside one, permitted development narrows and design scrutiny rises.
- Brookshill Drive and Grimsdyke Estate
- Harrow on-the-Hill / Harrow Park
- Harrow on-the-Hill / Harrow School
- Harrow on-the-Hill / Mount Park Estate
- Harrow on-the-Hill / Roxborough Park and The Grove
- Harrow on-the-Hill / Roxeth Hill
- Harrow on-the-Hill / South Hill Avenue
- Harrow on-the-Hill / Sudbury Hill
- Harrow on-the-Hill / Village
- Pinner / East End Farm
- Pinner / Moss Lane
- Pinner / Pinner High Street
- Pinner / Pinner Hill Estate
- Pinner / Pinnerwood Farm
- Pinner / Pinnerwood Park Estate
- Pinner / Rayners Lane
- Pinner / Tookes Green
- Pinner / Waxwell Close
- Pinner / Waxwell Lane
- Pinner / West Towers
- Pinnerwood Park Estate Conservation Area PART 1
- Stanmore and Edgware / Canons Park Estate
- Stanmore and Edgware / Kerry Avenue
- Stanmore and Edgware / Little Common
- Stanmore and Edgware / Old Church Lane
- Stanmore and Edgware / Stanmore Hill
- West Drive Conservation Area
Source: planning.data.gov.uk · Open Government Licence. Boundaries are checked at address level by the area report.
Article 4 directions in Harrow
Real · planning.data.gov.ukNo Article 4 geometry for Harrow appears in the national planning.data.gov.uk dataset, but the borough operates householder Article 4 directions within its conservation areas — removing permitted development rights so that even minor external alterations there need planning permission. Outside the conservation areas, normal PD rights generally apply; the borough's Residential Design Guide SPD sets the design expectations. Check the council's Article 4 register for the position at a specific address.
Source: planning.data.gov.uk · Open Government Licence. Checked at address level by the area report.
Prices: HM Land Registry UK House Price Index, March 2026 · Open Government Licence.
The planning route — PD or permission?
Permitted development (GPDO Class A) covers single-storey rear extensions up to 3m beyond the original rear wall on attached houses and 4m on detached, with a maximum height of 4m. The 'larger home extension' route extends this to 6m/8m through prior approval with neighbour consultation — but that larger route is not available in conservation areas.
In conservation areas, the basic 3m/4m single-storey rear PD allowance usually survives — what conservation-area status removes is side extensions and two-storey rear extensions. An Article 4 direction can remove more, but Camden's directions in Hampstead and Belsize target front-and-side appearance (solar panels, window changes, boundary treatments), not the single-storey rear allowance. Flats have no PD rights at all. Two-storey rear extensions in conservation areas always need full permission.
Practical rule across London: check the address first. If a conservation area or Article 4 direction applies, budget for a full householder application decided on design, neighbour daylight (the 45-degree test) and materials.
Permitted development limits — rear extension
| Single-storey rear depth — attached house | 3m beyond the original rear wall |
| Single-storey rear depth — detached house | 4m beyond the original rear wall |
| Larger extension via prior approval | 6m attached / 8m detached — not in conservation areas |
| Maximum height (single storey) | 4m |
| Two-storey rear in a conservation area | Full planning permission required |
| Flats | No permitted development rights |
Limits summarise the General Permitted Development Order (Schedule 2, Part 1) as it applies in London. In Harrow, a conservation area, Article 4 direction or listed status changes the route — the panels above show what is designated, and the address check confirms what applies at your property.
How much does a rear extension cost in Harrow?
| Cost per m² (low — straightforward site) | £3,000 |
| Cost per m² (expected) | £3,800 |
| Cost per m² (high — conservation spec, hard access) | £4,600+ |
| Typical build cost (12–18m² single storey) | £36,000 – £83,000 |
| Professional fees, surveys, party wall (add) | 10–18% of build |
Indicative London ranges calibrated from real project data. Conservation-area specifications (matching stock brick, lime mortar, bespoke glazing) and restricted rear access are the two biggest cost drivers. VAT not included.
Realistic timeline
| Design and drawings | 4–8 weeks |
| Planning decision (full application) | 8–12 weeks (8-week statutory target) |
| Prior approval route, where available | 42 days |
| Party wall agreements | 4–10 weeks (parallel) |
| Build | 3–5 months |
What catches people out in Harrow
National geometry does not currently hold Harrow's Article 4 directions — confirm conservation-area directions on the council's register as well as the free area report. Daylight and party wall issues remain routine hold-ups on semis.
Postcode districts in Harrow
HA1 centres on Harrow on the Hill — the Village, Harrow School and Harrow Park conservation areas, with Roxborough Park nearby.
HA1 — Harrow & Harrow on the Hill →HA5 is Pinner — the Pinner High Street, Moss Lane, Waxwell Lane and Pinnerwood Park Estate conservation areas, a remarkably intact old village core.
HA5 — Pinner →HA7 is Stanmore — the Stanmore Hill, Old Church Lane and Little Common conservation areas and the Grimsdyke Estate on the wooded ridge.
HA7 — Stanmore →HA2 covers South Harrow, Roxeth and the Rayners Lane conservation area — Metro-land suburbia at its most characteristic.
HA2 — South Harrow & Rayners Lane →HA3 runs from Wealdstone and Kenton up to Harrow Weald and the Grimsdyke ridge.
HA3 — Wealdstone, Kenton & Harrow Weald →Rear extension in Harrow, asked straight
Do I need planning permission for a rear extension in Harrow?
How much does a rear extension cost in Harrow?
What does the Residential Design Guide SPD cover?
Why might the free area report miss Article 4s?
Do I need a party wall agreement?
What applies at your address?
Borough-level rules only narrow it down. Enter a Harrow postcode for the live constraint check — conservation area, Article 4 and sold-price comparables, cited to source.
Free, and it stays free — no account, no payment. The check reads official geometry at the property's own coordinates; listed status, lease terms and the state of the building all sit outside it.
What else a rear extension in Harrow can need
Planning permission is one consent. A rear extension in Harrow can still need any of these, including when the planning route itself looks straightforward.
- Party wall surveyor in London — the works cut into a shared wall, sit on the boundary, or excavate near a neighbour.
- Listed building consent in London — the building is listed. Internal alterations can need consent as well as anything visible from the street.
- Licence to alter in London — the property is a leasehold flat. The freeholder's consent is separate from the council's.
Other projects in Harrow
Workable on the Edwardian terraces of Harrow and Wealdstone; the hilltop and Pinner conservation areas need a full application.
Side return extension in Harrow →Strong on the interwar semis of Pinner, Stanmore and Kenton under PD; the Harrow on the Hill and Pinner conservation areas scrutinise dormer design.
Loft conversion in Harrow →Uncommon on the borough's generous-plot stock; a full application with structural and ground-condition evidence is expected, more so in the conservation areas.
Basement extension in Harrow →Large suburban gardens in Pinner, Stanmore and Harrow Weald suit garden rooms under PD; the conservation areas can remove outbuilding rights — check the address.
Garden room in Harrow →Who to talk to about a rear extension in Harrow
A rear extension is a design-and-consent job first: someone has to draw it and take it through Harrow's planning route. The neighbour, condition and party wall side is separate work and a different profession — which is why there are two doors here, not one.
An extension is a design and consent job: the scheme, the planning application or prior approval, then the building-regs package.
- Residential design for extensions, lofts, basements and conversions
- Planning applications, pre-application advice and appeals
- Lawful development certificates
- Building-regulations and technical design drawings
- Conservation-area design and listed building consent applications
Excavating foundations within 3m of a neighbour is notifiable — the surveyor serves the party wall notices.
- Building surveys, RICS home surveys and defect diagnosis
- Party wall matters and awards
- Technical due diligence before a purchase
- Leasehold and major-works advisory (Section 20)
- Dilapidations and building pathology
Both practices are independent and separately regulated — Hampstead Chartered Surveyors & Building Consultancy is an RICS-regulated firm (Firm Reg. 923064); Hampstead Architecture and Planning are ARB-registered architects. We publish no fees for their work: each quotes directly for the job in front of them. Your enquiry goes to the practice you choose — we never pass your details to builders or other third parties.
Planning terms on this page
Conservation Area · Article 4 Direction · Permitted development · Full planning · Prior approval · Party wall · Designated land
Plain-English definitions · full planning glossary →
Planning Permission Checker provides planning and cost intelligence for early feasibility only. It is not legal, planning, valuation, architectural, structural, or surveying advice. All estimates are indicative and must be verified by qualified professionals before purchase, design, planning submission, or construction.
Cost estimates are indicative only — not a quotation. Final price depends on survey, specification, structure, access, party wall matters, VAT, professional fees, and contractor availability.
Planning outcomes are not guaranteed. Local planning authorities make final decisions.