How do I plan a house extension, step by step?
Plan an extension in seven steps: set a budget with a contingency, get a measured survey, agree the design, confirm the planning route, prepare building regulations and structural drawings, serve any party wall notices, then fix the build price and programme. Most projects take 2 to 4 months before work starts.
1. Set the budget and the brief
Decide what the extension must do and what you can spend, including a contingency of about 10%. Use our guide-price tool to check the size you want against the budget.
2. Survey and design
A measured survey records your house accurately. From it, two or three layout options are drawn so you can compare space, light and cost before choosing one.
3. Confirm the planning route
Permitted development, prior approval in England, or a householder application. Check conservation area status, Article 4 directions and earlier extensions first.
4. Building regulations and structure
Building regulations drawings, a specification and structural calculations turn the design into something buildable and approvable.
5. Neighbours and party walls
Talk to your neighbours early. In England and Wales serve party wall notices where the Act applies.
6. Fix the price and programme
With approved drawings, the build can be priced in writing and scheduled, including lead times for steel, glazing and kitchens.
7. Build, inspect and sign off
Building control inspects at key stages. At the end you receive the completion certificate, and any guarantees for windows, roofs and appliances from the manufacturers.
This guide is general information, not legal or professional advice for your property. Rules can change; we check the current position for your address at the survey.