How to Plan a Home Extension in 2026 (UK)
A successful extension is won or lost at the planning stage, long before a brick is laid. Get the brief and budget right, confirm whether you need planning permission or can build under permitted development, line up the right drawings and approvals, and vet at least three like-for-like quotes — and the build itself becomes the easy part. This guide walks you through every step, from first idea to Building Control sign-off, so you avoid the delays and cost overruns that catch most homeowners out.
How to Plan Your Home Extension
- Set your brief and budget. Write down what the extension must achieve, then set a realistic budget at £2,000–£3,000/m² plus a 10–15% contingency and VAT.
- Check planning vs permitted development. Many single-storey rear extensions are permitted development, but confirm the limits for your property and apply for a Lawful Development Certificate or full planning where needed.
- Appoint a designer or architect. An architect, architectural technologist or design-and-build firm turns your brief into drawings and handles the planning application.
- Get structural and Building Regs drawings. A structural engineer sizes the beams and foundations; Building Regs drawings are required for the build regardless of planning.
- Get and vet at least three quotes. Issue the same drawings and spec to three builders so the quotes are truly like-for-like, then check insurance, references and recent work.
- Sort Party Wall, insurance and the contract. Serve Party Wall notices where needed, confirm the builder's public liability cover, and agree a written contract with staged payments.
- Build and get it signed off. Building Control inspects at key stages; keep the completion certificate and warranties for future sale.
How Long Each Stage Takes
Budget 4–6 months from first idea to finished room for a typical single-storey extension that needs planning.
Get Extension Quotes From Vetted Builders
BestBuilders matches you with up to 3 vetted UK extension builders who quote on the same drawings and spec — so you can compare like-for-like with no surprises.
Common Questions
Related Guides
More extension how-to and cost guides.