How to build a house extension in England: step by step

1. Decide what you want, and keep a contingency

Start with how you want to use the space: a bigger kitchen, a downstairs bedroom, a home office. That decides the size, the shape and where the extension goes. Talk to a designer early — small changes on paper are cheap; the same changes on site are not.

Keep part of your budget aside for surprises. Older houses in particular hide things that only show up once walls are opened and foundations are dug.

2. Check whether you need planning permission

Many single-storey extensions are permitted development, which means no planning application is needed. In England, a single-storey rear extension can usually go up to 3 m deep on a semi-detached or terraced house and 4 m on a detached house, with height limits close to boundaries.

Under the Larger Home Extension scheme you can go up to 6 m (attached) or 8 m (detached) by notifying the council first. Your neighbours are told and can comment, and the council decides within 42 days.

Permitted development does not apply to flats and maisonettes, and it is more limited in conservation areas, National Parks, National Landscapes (AONB) and World Heritage Sites. Councils can also remove these rights in some areas with an Article 4 direction. Listed buildings need listed building consent for almost any change.

• Use our free planning checker for a first answer: planning checker
• For certainty, apply for a Lawful Development Certificate — proof that the work was lawful, useful when you sell.

3. Get drawings and a structural design

An architect or architectural technician turns the idea into drawings: plans, elevations and a section through the building. If you are removing walls, adding steel beams or building on difficult ground, a structural engineer designs the structure and provides calculations.

These drawings are what the council, Building Control and your builder all work from, so the clearer they are, the more accurate the quotes will be.

4. Building Regulations approval

Every extension needs Building Regulations approval, even when no planning permission is needed. Building Regulations cover structure, fire safety, insulation, drainage, ventilation and electrical safety.

You can make a full plans application, where the drawings are checked and approved before work starts, or give a building notice, where there is no plan approval and the work is checked as it goes. Full plans gives more certainty on bigger jobs.

Building Control — from the council or a registered building control approver — inspects the work at key stages such as foundations, drainage and completion. At the end you receive a completion certificate. Keep it: buyers' solicitors will ask for it.

5. Tell your neighbours: party wall notices

If the work touches a shared wall, involves building a new wall on the boundary, or digs foundations close to your neighbour's building, the Party Wall etc. Act 1996 requires written notice before work starts — two months for work on a party wall and one month for new boundary walls or excavation.

• Create the notice with our free generator: party wall notice
• Read the full guide: party wall rules explained

6. Choose a builder carefully

Get written, itemised quotes so you can compare like with like — what is included, what is not, and what happens if something unexpected is found.

Check who you are hiring: look the company up on Companies House, ask for proof of insurance (public liability and employers' liability), and ask to see or speak to past clients.

Agree staged payments linked to stages of the work, and avoid paying large sums up front. Put the agreement in writing, including the programme, the price and how changes are handled.

Under the Construction (Design and Management) Regulations 2015, a homeowner's health and safety duties normally pass to the contractor — another reason to use a builder who plans the work properly.

7. During the build

A typical extension runs through set-up and protection of the site, foundations (inspected by Building Control), walls up to roof level, the roof, windows and doors, first fix of electrics and plumbing, plastering, second fix, and finishing.

Gas work must be done by a Gas Safe registered engineer, and electrical work by a qualified electrician who can certify it. Keep the conversation going with your builder and your neighbours — most disputes start with surprises.

8. Finishing properly

Walk round with your builder and list anything that needs putting right (snagging). Before the final payment, make sure you have the Building Control completion certificate, the electrical installation certificate and, if gas work was done, the Gas Safe certificate, plus any product warranties.