BOROUGH · PROJECT

Rear extension in Barnet

Planning permission, real costs and what actually gets approved

Often not, for houses outside Barnet's conservation areas — permitted development covers a single-storey rear extension up to 3m on an attached house or 4m on a detached one, and the larger 6m/8m prior-approval route is available too. Inside one of Barnet's Article 4 conservation areas, that allowance is generally removed altogether: the borough's directions reach even minor external changes, not just larger or two-storey additions, so a full householder application is the default there. Check your specific address before assuming either way.

Generous interwar plots make rear and wrap-around extensions feasible; the Article 4 conservation areas (HGS, Mill Hill, Totteridge) need a full application.

Barnet is really two different planning environments stitched into one borough. Across most of its interwar semi-detached and Edwardian streets — wide plots, side access, real garden depth — permitted development carries a rear extension without a fight, and the borough is genuinely one of outer London's most active markets for this kind of project. Its Article 4 conservation areas are the opposite case: there, even a modest single-storey rear addition typically needs a full application rather than a permitted-development sign-off, because Barnet's directions reach further than the standard conservation-area baseline. Which environment a given house sits in is exactly what the address-level check is for.

CHECK

What actually applies in Barnet

Conservation areas in Barnet

Real · planning.data.gov.uk

Every designated conservation area in Barnet from the official dataset — inside one, permitted development narrows and design scrutiny rises.

College Farm
Finchley Church End
Finchley Garden Village
Glenhill Close
Golders Green
Hampstead Garden Suburb
Hampstead Village (Heath Passage)
Hendon Church End
Hendon The Burroughs
Mill Hill
Monken Hadley
Moss Hall Crescent
Railway Terraces
The Watling Estate
Totteridge
Wood Street

Source: planning.data.gov.uk · Open Government Licence. Boundaries are checked at address level by the area report.

Article 4 directions in Barnet

Real · planning.data.gov.uk

Barnet's Article 4 directions are recorded in the national dataset as 48 separate parcels, the great majority of them householder directions that remove permitted development rights across its conservation areas — Hampstead Garden Suburb, Finchley Church End, Mill Hill, Monken Hadley, Totteridge, Wood Street, Moss Hall Crescent, Glenhill Close and Finchley Garden Village among them — so even minor external alterations there need a planning application. A separate direction controls small house-to-HMO conversions, and others cover agricultural land. Use the area report, or the council's Article 4 register, for the direction that applies at a given address.

Source: planning.data.gov.uk · Open Government Licence · 48 directions recorded. Checked at address level by the area report.

Average house price

£595,567

Annual change

-5.2%

Prices: HM Land Registry UK House Price Index, November 2025 · Open Government Licence.

ROUTE

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.

COST

What it really costs

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.

TIME

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

WATCH

What catches people out in Barnet

The classic Barnet mistake is assuming permitted development applies because it does across most of the borough — then discovering the house sits inside one of the Article 4 conservation areas, where even a modest single-storey rear extension needs a full application and drawings sketched for a permitted-development scheme have to be reworked for design scrutiny. On the wider unconstrained stock, the 45-degree daylight test and party wall notices on the shared flank are the more routine hold-ups.

FAQ

Rear extension in Barnet, asked straight

01

Can I build a rear extension in Barnet without planning permission?

On most of Barnet's interwar and Edwardian housing stock, yes — a single-storey rear extension within the 3m/4m permitted development limits, or up to 6m/8m via prior approval, can proceed without a full application. The exception is Barnet's Article 4 conservation areas, where permitted development rights are removed and even a modest rear addition needs planning permission. Check your specific address before assuming either way.

02

How much does a rear extension cost in Barnet?

Realistic 2026 budgets run £3,000–£4,600 per m², or roughly £36,000–£83,000 for a typical 12–18m² single-storey rear extension, before VAT and professional fees. Barnet's larger plots and easier site access remove one of the two biggest cost drivers this project usually carries — restricted rear access — though conservation-area specification still pushes costs toward the top of the range in the borough's protected pockets.

03

Do I need a party wall agreement for a Barnet rear extension?

Very likely, if you share a wall with a neighbour — and much of Barnet's stock is semi-detached, so that typically means one shared flank rather than the two a mid-terrace faces. Excavating foundations within 3m of the boundary or building on the line of junction triggers the Party Wall etc. Act 1996 regardless of your planning route, so serve notice early.

04

How do I find out if my Barnet house is in an Article 4 conservation area?

Run your postcode through the area check — it tests your coordinates against the official conservation-area and Article 4 geometry registered for Barnet and shows which designation, if any, applies to your address before you commission drawings.

05

Can I extend a Barnet semi-detached house on both the rear and the side?

Often, yes, on unconstrained plots — Barnet's wider interwar frontages and side access are exactly what make combined rear-and-side 'wrap-around' schemes more achievable here than on a tight inner-London terrace. A wrap-around almost always exceeds the single-storey rear permitted development limits, though, so treat it as a full householder application rather than assuming permitted development covers it.

CHECK

What applies at your address?

Borough-level rules only narrow it down. Enter a Barnet postcode for the live constraint check — conservation area, Article 4 and sold-price comparables, cited to source.

Check a Barnet address

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.