Adding a home extension is one of the best ways to improve your living space without the stress of moving. But while the end result can be life-changing, the journey there isn’t always smooth — especially if you fall into one of the common traps we see far too often.

At XMX London, we’ve handled all kinds of house extensions across the city, and we know exactly where things tend to go wrong. Here are five common mistakes to avoid if you want your extension to go smoothly from start to finish.

1. Starting Without Proper Plans

We get it — it’s tempting to get started quickly. But skipping the planning stage is a sure way to hit delays and budget issues later. Always work with an architect or designer to properly map out the space, layout, and function. A clear plan now avoids big regrets later.

2. Underestimating the Budget

One of the biggest mistakes is setting a budget that’s too tight — with no room for the unexpected. And trust us, the unexpected happens. Old pipework, hidden damp, or tricky access to your property can all add to the cost. Always keep a contingency fund (we recommend 10–15% extra).

3. Ignoring Planning Permission Rules

Assuming your extension falls under permitted development can be a costly error. Every borough has different regulations, and what’s fine in one area might be rejected in another. Always check before building — or work with a team who can do that for you.

4. Choosing the Cheapest Quote

Everyone wants a good deal — who doesn’t? But chasing the lowest price without digging into who’s actually doing the work can end up costing you more in the long run.
We’ve seen it before: jobs rushed, poor materials, timelines slipping, and suddenly that “bargain” build turns into a string of headaches.
Instead, go with someone you can trust. Check out their previous work, ask for references, see what past clients are saying. A reliable builder won’t shy away from questions — and that bit of homework pays off big time.

5. Best Use of Space

It’s great to have a lot of space but if it gets in the way of efficiency and functionality then it’s not designed around the way you live and could get in the way

Where does the morning light hit? How do you move through the rooms? Do things flow or feel disconnected?
We’ve walked into plenty of extensions that look nice on paper but don’t quite work in real life — because no one stopped to think about how the space would actually be used.

6. Wrapping it Up

A good extension shouldn’t just add space — it should make your life easier.
And avoiding the common traps we’ve mentioned here? That’s half the battle won.
Getting the right team really helps, asking the right questions and have a vision think beyond just walls and floors. That’s how you end up with seamless results.

Got something in mind? We’re always happy to chat. No pressure, no hard sell — just honest advice and a builder who knows their way around London homes.