How to plan a house extension, step by step

From the first idea to the final sign-off: the steps to planning an extension, in the right order, and where most projects go wrong.

A good extension starts long before anyone digs a foundation. Getting the steps in the right order saves money, avoids redesigns and gets you building sooner. Here’s how we plan extensions with our clients across Cambridgeshire, Bedfordshire and Milton Keynes.

Start with how you want to live rather than how big the extension should be. A bigger kitchen-diner, a downstairs bedroom, a home office or a utility room all lead to different designs. Make a list of must-haves and nice-to-haves before you speak to anyone.

Build costs, design fees, approvals, kitchens and finishes all add up. A builder can give you a realistic range at the first site visit, before you spend anything on detailed drawings. It’s much easier to design to a budget than to cut a design back later.

• Whether it can be permitted development or needs planning permission
• Conservation areas, listed buildings and any planning conditions on your home
• Drains, trees, boundaries and access for the build
• Whether Party Wall notices will be needed

Sketch options come first, then detailed plans for planning or a Lawful Development Certificate. Once approved, building regulations drawings and structural calculations set out exactly how it will be built, which is what gives you a firm, itemised price.

On site, building control inspects the work at key stages. When it’s finished you should receive a completion certificate, which you’ll need when you sell. Keep it with your planning or LDC approval and any guarantees.