1. Decide what you need the space for
Start with how you want to live, not the size of the box. A bigger kitchen-diner, a downstairs bedroom and a home office lead to very different designs. Note what must stay (light, garden, access) and set a realistic budget with a contingency for the unexpected.
2. Get a design
An architect, architectural technologist or experienced designer can turn your brief into drawings. Ask early whether the design is likely to fall under permitted development or will need a planning application, because that can shape the design.
3. Sort out planning
- Permitted development: no application needed, but consider a lawful development certificate for peace of mind.
- Prior approval: for larger single-storey rear extensions, your council must approve the extension before you build.
- Planning application: needed for anything outside the limits, and for flats, listed buildings and some protected areas.
4. Plan for Building Regulations
Most extensions need Building Regulations approval. Structural calculations from an engineer are often needed for steels, beams and foundations. Decide who will deal with Building Control: you, your designer or your builder.
5. Talk to your neighbours
If you will be working on a shared wall, building on the boundary or digging foundations close to a neighbour’s, you may need to serve a party wall notice. Talking to neighbours early often makes the formal process easier.
6. Get quotes from builders
Send the same drawings and specification to at least three builders so their quotes are comparable. Check membership of a trade body, insurance and recent similar work. Our guide to choosing a builder covers what to look for, and you can find extension builders here.
7. Agree a contract and payment stages
Put the scope, price, timescale and stage payments in writing. Avoid paying large sums upfront, and keep a final amount back until the work is complete and signed off.
8. Build, inspect and sign off
Building Control will inspect at key stages, such as foundations and drainage. At the end, make sure you receive a completion certificate, plus electrical and gas certificates and any guarantees. Keep them safe: buyers’ solicitors will ask for them.
Last updated: 14 October 2026
