Planning an extension

An extension is usually the biggest home improvement people take on. Done well, it can give you the space you need without moving. This guide covers the main decisions and the rules that apply, so you can have a better conversation with the builders you speak to.

Single-storey or two-storey?

Single-storey extensions are the most common. They're often used for a bigger kitchen, an open-plan living and dining space or a utility room. They're usually quicker to build and less disruptive.

Two-storey extensions add space upstairs as well, such as an extra bedroom or bathroom. The cost per square metre is often lower than building two separate single-storey spaces, because you share the foundations and roof. They're more likely to need a planning application.

If you're not sure, think about what you're short of. If it's living space, single-storey may be enough. If it's bedrooms, two-storey is worth exploring.

Planning permission

Many extensions can be built under "permitted development" rights, without a planning application. There are limits on things like how far the extension comes out from the original house, its height, how close it is to a boundary and how much of your garden it covers. The rules are stricter for two-storey and side extensions.

Permitted development rights don't apply, or are reduced, if:

• your home is a flat or maisonette
• your home is listed, or in a conservation area or other protected area
• the rights have been removed for your property, for example by a condition on an earlier planning permission

If you're relying on permitted development, you can apply to your council for a Lawful Development Certificate. It isn't required, but it confirms the work is lawful, which helps when you come to sell.

The Planning Portal has an interactive guide to extensions, and your council's planning team can advise. Rules differ in Wales and Scotland.

Building Regulations

Almost all extensions need Building Regulations approval, whether or not they need planning permission. Building Regulations cover things like foundations, structure, fire safety, insulation, drainage and ventilation.

You or your builder apply to building control, either at your council or a registered private building control approver. They inspect the work at key stages and issue a completion certificate at the end. Keep it safe: you'll need it when you sell.

Some parts of the job, such as electrics, windows or heating, may be signed off by an installer registered with a Competent Person Scheme. Your builder should tell you how each part will be certified.

Party walls and neighbours

If you're building on or near a wall you share with a neighbour, or digging foundations close to their home, the Party Wall etc. Act 1996 may require you to give them notice. GOV.UK has guidance on how this works. Talking to your neighbours early, even when you don't strictly need to, can save a lot of trouble.

Drawings and design

Most extensions need drawings, both for planning (if required) and for Building Regulations. Some builders offer design and build, and some people use an architect or architectural technician separately. A structural engineer may be needed for steel beams or where walls are removed.

Questions to ask before you agree a quote

• What exactly is included, and what isn't? (Kitchen, flooring, decorating, electrics and plumbing are often separate.)
• Who will deal with planning and building control, and are their fees in the quote?
• How will each part of the work be certified?
• What insurance-backed guarantee do you offer, and who provides it?
• How long will the work take, and what happens if it overruns?
• How will payments be staged? Avoid paying large sums up front.
• Can I see examples of similar extensions you've built, and speak to past customers?

Getting two or three quotes for the same specification makes it much easier to compare like for like.