Can you build a 6 or 8 metre rear extension without planning permission?

What happens, step by step

1. You send the council a written description giving the depth beyond the original rear wall, the maximum height and the eaves height, a plan of the site, the addresses of adjoining premises, your contact address and the fee (A.4(2)). If the extension joins an earlier one, the figures are for the two together.
2. The council serves notice on each adjoining owner or occupier, with a date for comments at least 21 days away, not counting public holidays (A.4(5), A.4(16)).
3. If no adjoining owner or occupier objects, prior approval is not required. If one does, the council decides the impact on the amenity of all adjoining premises, not only the objector's (A.4(7), A.4(9)).
4. You may start on written notice that approval is not required, on written approval, or 42 days after the council received your information if it has not told you its decision (A.4(10)).
5. You build what you described or what was approved, unless the council agrees a change in writing (A.4(11)).

The development must not begin before the occurrence of one of the following ... the expiry of 42 days following the date on which the information referred to in sub-paragraph (2) was received by the local planning authority without the local planning authority notifying the developer as to whether prior approval is given or refused.

What the council can consider

The prior approval question is the impact on the amenity of adjoining premises, and it arises only if a neighbour objects. Planning practice guidance says a council cannot consider any other matters on a prior approval. The council can still refuse if the extension does not meet the Class A limits, or if you have not given enough information to show that it does (A.4(3)). It can approve with conditions reasonably related to neighbours' amenity (A.4(12)). A refusal can be appealed under section 78 of the Town and Country Planning Act 1990.

The rest of Class A still applies

The larger limit replaces only the depth in A.1(f). The extension must still be a single storey no more than 4 metres high, keep its eaves to 3 metres within 2 metres of a boundary, stay within the 50% ground cover limit, match the materials of the house, and include no raised platform. The A.4 procedure applies only to development otherwise permitted by Class A (A.4(1)), so 42 days of silence do not make an extension lawful if it breaks another limit. Rear extensions sets out the full list.

Where it does not apply

Paragraph A.1(g) applies only to a house "not on article 2(3) land nor on a site of special scientific interest". Article 2(3) land includes conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites. There the limit stays at 3 metres, or 4 metres on a detached house. Class A does not apply to flats, or to houses created under the change of use rights in Part 3 or built under Part 20 (A.1(a) and (l)).

What this means for you

Measure from the original rear wall and run your figures through the permitted development checker. A neighbour's objection is what turns the notification into a decision on amenity, so the neighbours on each side are worth speaking to before the council's letter arrives.

Getting certainty

Prior approval deals with the neighbours' amenity. It is not a lawful development certificate, and only a certificate under section 192 of the Town and Country Planning Act 1990 carries the conclusive presumption of lawfulness in section 192(4). If you want that confirmation for the whole extension, a certificate for proposed works costs £274 from 1 April 2026 and the council has 8 weeks to decide.