Do you need planning permission for an extension?

Not every extension needs full planning permission. A lot of common single-storey rear extensions fall under permitted development rights in England, which let you build without a full planning application - but the limits are specific, and it's easy to assume you're covered when you're not.

When permitted development typically applies

As a rough guide (rules differ for flats, listed buildings, and conservation areas, and have changed over time, so always verify current rules for your property):

• Single-storey rear extensions are commonly allowed up to 3m beyond the rear wall for a terraced or semi-detached house, or 4m for a detached house (extended limits are available via a "prior approval" notification process).
• Height limits typically apply - commonly around 4m maximum for a single-storey rear extension.
• Materials are generally expected to be similar in appearance to the existing house.
• Side extensions and two-storey extensions have their own, generally stricter, limits.

When you'll need full planning permission

• You're in a conservation area, an Area of Outstanding Natural Beauty, or your building is listed.
• The extension exceeds permitted development size or height limits.
• Your property is a flat or maisonette - permitted development rights for extensions generally don't apply.
• A previous extension has already used up your permitted development allowance.

Get it confirmed, don't assume it

Even where permitted development likely applies, it's strongly worth getting a Lawful Development Certificate from your local council - a formal, low-cost confirmation that protects you if a neighbour complains or you come to sell the house later. An experienced local builder or architect will usually have a good instinct for whether your plans are within permitted development limits, but the council's confirmation is the only thing that removes doubt entirely.