Often not. Under permitted development you can build a single-storey rear extension up to 3 m deep on a terrace or semi (4 m detached), or 6 m and 8 m with prior approval, and a rear dormer loft up to 40 m³ on a terrace or 50 m³ on a semi, without planning permission. Flats, listed buildings, conservation areas and Article 4 streets always need an application.
What permitted development allows
Permitted development (PD) is a national set of rights that let homeowners extend without applying for planning permission, provided the work stays inside fixed limits. For a single-storey rear extension the limits are 3 m deep on a terraced or semi-detached house and 4 m on a detached house, no higher than 4 m overall, with eaves no higher than 3 m if within 2 m of a boundary. The extension plus any previous additions must not cover more than half the garden.
The larger home extension scheme extends these depths to 6 m and 8 m respectively through a prior approval notification. The council writes to your neighbours, and if none object within 21 days, the extension is approved within 42 days.
For lofts, PD allows up to 40 m³ of additional roof volume on a terrace and 50 m³ on a semi or detached house, as long as the dormer is at the rear or side, sits below the ridge and is set back at least 20 cm from the eaves.
When you always need planning permission
Flats and maisonettes have no permitted development rights at all. Listed buildings need listed building consent for almost any change. In conservation areas, side extensions, rear extensions of more than one storey, cladding and roof extensions are excluded from PD. Many London boroughs have also issued Article 4 directions that remove PD rights street by street, particularly in Islington, Camden, Hackney, Kensington & Chelsea and Richmond.
Double-storey rear extensions beyond 3 m, any extension forward of the principal elevation, and extensions on houses that have already been enlarged under PD will also require an application.
The planning application process
A householder planning application costs £258 in 2026 and takes 8 weeks from validation to decision. You will need existing and proposed plans and elevations, a site location plan, a design and access statement in conservation areas and, for basements or sites near rivers, additional technical reports. Neighbours are consulted for 21 days.
Even when PD applies, we recommend applying for a Lawful Development Certificate (£129). It confirms in writing that the work was lawful, which your buyer's solicitor will ask for when you sell.
Planning permission is not Building Regulations
Planning deals with the appearance and impact of what you build; Building Regulations deal with how it is built. Every extension and loft needs Building Regulations approval regardless of whether planning permission is needed. ICV Buildings handles both, with building control inspections at foundations, damp-proof course, structure, insulation and completion.
Frequently asked questions
- How long does planning permission take in London?
- Eight weeks for a householder application, plus one to two weeks for validation. Add four to eight weeks beforehand for drawings. Realistically, plan on three to four months from first sketch to approval.
- What happens if I build without permission?
- The council can issue an enforcement notice requiring you to alter or demolish the work, and unauthorised work makes a house hard to sell or mortgage. A retrospective application is possible but not guaranteed.
- Who prepares the drawings?
- ICV Buildings works with partner architects who prepare planning and Building Regulations drawings as part of our design-and-build service, so you deal with one team from sketch to handover.