So, you’ve decided your London home needs more space. Maybe the kitchen feels tight, maybe the kids are growing and sharing a room isn’t working anymore, or maybe you just want to bring more light and life into your everyday living. Whatever the reason, an extension sounds like the perfect solution — and in many cases, it is.

But before you call in the builders or start sketching ideas on the back of an envelope, it’s worth knowing a few key things. As a company that’s been building extensions across London for years, here’s what we at XMX London think every homeowner should understand before they dive in.

1. Every Borough Has Its Own Rules

This might come as a surprise, but not all planning permissions are created equal — especially in London. What flies in Wandsworth might get flagged in Camden. Different boroughs have their own rules — some won’t let you build too close to the neighbour’s fence, others might get picky about the height or even the type of bricks you’re planning to use. It’s not always logical, but it’s the reality of building in London.

Best bet? Have a chat with someone who’s been through it a few dozen times — like us. We’ve seen what Camden might let through that Ealing won’t touch, and trust me, it pays to know this stuff before you start spending money.

2. Permitted Development Isn’t Always Enough

Many people think they can build under permitted development rights — and in some cases, that’s true. But don’t assume. It depends on the type of extension, your property type, whether it’s in a conservation area, and a whole list of other things. A good builder will help you figure this out early on, before you spend money on drawings that won’t be approved.

3. Budget for the Unexpected

We won’t sugar-coat it: no matter how well you plan, building work can uncover surprises. Old pipework, hidden structural issues, or even awkward access for deliveries — especially in tight London streets — can all affect costs.

Set a realistic budget, then add a buffer. You’ll thank yourself later.

4. Design Makes a Huge Difference

It’s not just about how big your extension is — it’s about how it works. Natural light where you need it, storage where it makes sense, and a layout that actually fits the way you live. We’ve seen big extensions that feel cramped, and smaller ones that feel amazing. That’s the difference design makes. You want someone who gets that — not just someone who shows up with a toolbox.

5. Choose a Builder Who Knows London

This isn’t the countryside. Access can be tricky, neighbours are close, and the buildings themselves can be 100+ years old. You need a team who knows how to handle London properties — not just technically, but with respect for the area and your neighbours.

At XMX London, we’ve built everything from sleek rear extensions to full wraparounds, always tailored to the property and the people living in it.

Thinking about extending?

Let’s have a chat. No pressure, no jargon — just honest advice to help you get it right from the start.