Do I Need Planning Permission for an Extension? (2026)

Last updated: June 2026 · Verified against current government planning rules and North London council records.

Do I need planning permission for an extension in 2026?

Quick answer: Not always. A single-storey rear extension up to 3m deep on an attached house, or 4m on a detached house, is usually permitted development. You can reach 6m or 8m through the larger-home prior approval process, which costs £249. Conservation areas, Article 4 directions and larger or two-storey schemes need full planning permission.

Plenty of North London homeowners assume any extension means a planning battle. It often does not. The permitted development rules give meaningful freedom for single-storey rear extensions, and a prior approval route extends that freedom further still. But the limits are specific, and the moment you go two storeys, wrap around the side, or live in a restricted area, a full application becomes necessary. This guide sets out exactly when you are clear and when you must apply in 2026.

What size extension can I build under permitted development?

The depth allowance depends on your house type and whether you use the prior approval route. The table summarises the single-storey rear limits.

House type	Standard PD depth	With larger-home prior approval
Terraced or semi-detached (attached)	Up to 3m	Up to 6m
Detached	Up to 4m	Up to 8m

Height limits also apply: a single-storey rear extension must not exceed 4m to the ridge, or 3m if within 2m of a boundary, and eaves must not exceed 3m near boundaries. The larger-home prior approval scheme, costing £249, lets you go to the deeper 6m or 8m figures provided neighbours are consulted and do not object on valid grounds — it is a lighter-touch process than a full application, not an automatic right.

When does an extension definitely need planning permission?

Several scenarios remove permitted development entirely. A two-storey extension beyond modest limits, any extension forward of the principal elevation, a side extension wider than half the original house, building that would cover more than half the garden, or any work to a flat or listed building all require a full application. Critically, permitted development rights are frequently withdrawn in conservation areas and under Article 4 directions, which blanket roughly a quarter of North London streets. In those areas even a small rear extension that would be permitted elsewhere needs planning permission. Always check your address against the council's records before assuming permitted development applies.

What does the larger-home prior approval process involve?

For deeper single-storey rear extensions — beyond 3m attached or 4m detached, up to 6m or 8m — you use the neighbour consultation scheme, also called larger-home prior approval. You notify the council with details and drawings, paying £249. The council writes to adjoining neighbours, who have 21 days to object. If none object on legitimate grounds relating to amenity, the council issues prior approval and you may build. If a neighbour objects, the council decides whether the impact is acceptable. It is faster and cheaper than a full householder application (which costs £548 plus the £91.02 Planning Portal charge), but it only covers single-storey rear depth — not height, two storeys or side extensions.

Is a Lawful Development Certificate worth it for an extension?

If your extension is permitted development, a Lawful Development Certificate (Proposed) at £274 is the council's written confirmation that no planning permission is needed. It protects you against a future dispute and, more importantly, reassures buyers' solicitors when you sell — conveyancers routinely ask for proof that an extension was lawful. Spending £274 to remove that risk is sensible insurance against a stalled sale years later. It is separate from building regulations, which every habitable extension needs regardless of planning status, typically costing £500–£1,500 for building control sign-off.

What about side and wraparound extensions?

Side extensions follow tighter rules than rear ones. Under permitted development a single-storey side extension must be no wider than half the width of the original house, no higher than 4m, and single-storey. A wraparound extension — combining a rear and a side return into an L-shape — almost always exceeds permitted development because it stacks two allowances together, so it typically needs a full planning application even when each part might individually qualify. Side returns on Victorian terraces, hugely popular for widening a galley kitchen, frequently need permission for this reason. If your design turns the corner of the house, assume you are applying rather than relying on permitted development.

What height limits apply to a permitted development extension?

Height is as important as depth, and it catches people who focus only on how far back they can build. A single-storey rear extension must not exceed 4m in height overall, and crucially must not exceed 3m in height if any part sits within 2m of a boundary — the typical situation on a narrow North London plot. Eaves height is capped at 3m within 2m of a boundary. The materials must be similar in appearance to the existing house, and on designated land such as conservation areas the deeper prior-approval depths are not available at all. These height rules are why a flat-roofed extension is often easier to keep within permitted development than a pitched one near a boundary.

Frequently asked questions

How deep can a rear extension be without planning permission?

Up to 3m on an attached house or 4m on a detached one under standard permitted development, extendable to 6m or 8m through the larger-home prior approval process at £249. Beyond those figures, a full householder planning application is required.

Do two-storey extensions ever count as permitted development?

Only within tight limits, and they are excluded entirely in conservation areas and many Article 4 zones. Most two-storey rear and side extensions in North London need a full planning application because they exceed the modest permitted development thresholds for upper-floor additions.

How long does extension planning take?

A full householder application carries an 8-week determination target. The larger-home prior approval process runs on a 42-day timetable including the 21-day neighbour consultation. Permitted development with a Lawful Development Certificate is quicker, as the council only confirms lawfulness rather than judging merit.

Does an extension always need building regulations?

Yes. Every habitable extension requires building regulations approval covering foundations, structure, insulation, drainage and fire safety, completely separate from planning permission. Building control inspects the work and issues a completion certificate, typically for £500–£1,500. You can be permitted development and still need full building control sign-off.

Knowing whether you need permission shapes your whole budget and timeline. See current build figures on our extension cost North London page, or book a free site visit and we will check your permitted development rights and Article 4 status before designing — The Extension Company, Cockfosters, 020 3051 9430.