An extension is one of the biggest jobs most people ever do to their home — and one of the most rewarding. Whether it's more room for a growing family, a proper kitchen-diner, or finally giving yourself a downstairs office, an extension can transform how a house works without the upheaval of moving. But it's also a project with real decisions to get right before a single brick goes down. Here's a plain-English run-through of what's actually involved.
Do you need planning permission?
Not always. Many single-storey rear and side extensions fall under Permitted Development, which means you can build without a full planning application, provided you stay within set limits on height, depth and how much of your garden you cover. Roughly speaking, a single-storey rear extension on a semi-detached or terraced house can usually go up to 6 metres without needing planning permission (8 metres for detached homes, under the extended rules), though there are conditions around materials, roof pitch and boundary proximity.
Go beyond those limits, live in a conservation area or listed building, or want a two-storey extension, and you'll need a full planning application through your local council. It's always worth checking with the council — or asking your builder or architect — before you commit to a design, since it shapes everything that follows.
Building Regulations approval
This one's easy to overlook but it isn't optional: even a Permitted Development extension needs Building Regulations sign-off. This covers the things you can't see once the plaster's up — structural stability, insulation, fire safety, drainage and ventilation. Building control will inspect the work at key stages (foundations, damp-proofing, structural steels, completion) and issue a completion certificate at the end. Keep that certificate safe; you'll need it if you ever sell the house.
The Party Wall Act
If your extension involves work on or near a shared boundary or party wall — which most rear and side extensions do — you'll likely need to serve a Party Wall Notice to your neighbours under the Party Wall etc. Act 1996. It sounds more daunting than it usually is: it's a formal way of giving neighbours notice and agreeing how the work will be carried out, and in most cases a friendly conversation beforehand smooths the way considerably.
What actually happens on site
Every extension follows roughly the same sequence, even if the scale differs:
- Groundworks & foundations — digging out and pouring foundations, often the muddiest and least exciting part, but it's the bit everything else depends on.
- Structure — blockwork or brickwork walls go up, along with any structural steels needed to open up the space.
- Roof — timbers, felt and covering go on, and the extension becomes weathertight.
- First fix — plumbing, wiring and stud walls go in before anything is closed up.
- Plastering & second fix — walls are finished, and sockets, radiators, doors and fittings go in.
- Decoration & snagging — the final coat of paint, and a walk-through to check everything's exactly as it should be.
A typical single-storey extension takes somewhere between 10 and 16 weeks from breaking ground to finished room, depending on size, weather and how much is being altered inside the existing house too.
Choosing the right builder
This is the decision that matters most. Look for a builder who visits and properly discusses your plans rather than quoting from a phone call, gives you a clear itemised quote rather than a vague number, and can show you finished work similar to what you're planning — ideally on properties like your own. Ask how they handle Building Regulations and Party Wall matters, who will actually be on site day to day, and how they'll keep you updated as the job progresses. That's the difference reliable builders make — the process should feel manageable, not like something you have to survive.
Living through the build
It's worth being realistic: even a well-run extension brings some disruption — noise, dust, and a bit of coming and going. Setting up a temporary kitchen if yours is affected, agreeing working hours upfront, and having a clear point of contact for questions all make a real difference to how smooth it feels day to day.
If you're thinking about extending your own home in Cambridge or the surrounding area, we'd love to have a chat. We'll visit, listen to what you have in mind, and give you a clear, honest quote — no pressure, just a proper conversation.