Right, so you’ve decided to finally get that extension sorted, or maybe the loft’s been sitting there unused for years and you’ve had enough. Either way, the next step is the scary one: picking who actually does the work. London’s absolutely full of building contractors in London, and I mean full, you can’t scroll five minutes on Instagram without an ad for one. Problem is, half of them look identical on paper. Same glossy photos, same “we’re fully insured” line, same five-star reviews that may or may not be genuine. So how do you actually tell who’s worth your money and who’s going to leave you with a half-finished kitchen extension and a phone that stops ringing? Here’s what I’d actually check, based on the mistakes I’ve seen people make (and honestly, one or two I’ve made myself).
1. Ask to actually see the insurance, not just hear about it
Everyone says they’re insured. It’s practically a script at this point. What you want is the actual document, public liability cover, employer’s liability if they’ve got a team, and ideally proof they’re registered with something like the FMB. If a contractor gets weirdly evasive when you ask for paperwork, that’s not a red flag, that’s a flashing neon sign. Just walk.
2. Don’t trust a portfolio unless it matches what you’re doing
Here’s a thing nobody tells you: a company can have a gorgeous website full of finished kitchens and still have never touched a loft conversion in their life. If you want a side return extension, ask them straight up for side return jobs they’ve done recently, not just “extensions” in general. Push for the actual address or at least the area if they won’t give you the full thing. Anyone confident in their own work won’t blink at this.
3. Get the quote in writing, every single time
I don’t care how nice the bloke on the phone sounds. A verbal quote is worth nothing the second work starts and someone says “oh that wasn’t included.” Get it written down, materials, labour, rough timeline, and anything that’s a provisional sum rather than a fixed cost. It’s not about distrust, it’s just common sense, and it protects you both if things go sideways later.
4. Check if they’ve actually got architects and engineers in-house
This is one people skip because it sounds boring, but it matters more than most of the flashy stuff. The whole point of going with a design and build company instead of hiring an architect, then a structural engineer, then a builder separately, is that everyone’s supposedly talking to each other. So check that’s actually true. Are the architects in-house, or are they just farmed out to whoever’s free that week? You’ll notice the difference the moment something unexpected turns up on site.
5. Be suspicious of anyone who promises it’ll be quick
London houses are old, weird, and full of surprises. Party wall agreements alone can eat weeks if your neighbour’s slow to respond. Planning can drag. Access to the property might be a nightmare depending on where you are. So if someone tells you your double storey extension will be done in eight weeks flat, I’d raise an eyebrow. The contractors who give you a slightly less exciting but more honest timeline are usually the ones who know what they’re doing.
6. Actually ring the references, don’t just read the reviews
Reviews are fine, but everyone’s got a few good ones sitting on their site, that’s not hard to arrange. What you want is two or three past clients you can actually call and ask real questions. Not “were you happy,” but “what went wrong, and how did they deal with it.” Something always goes wrong on a build, always. It’s how they handled it that tells you what you’re actually dealing with.
7. Ask what happens after they’ve been paid
This bit gets forgotten constantly. What’s the warranty on the structural work itself? What about windows, roofing, the electrics? A decent company sticks around if a problem crops up six months later. The ones who go quiet the second the final invoice clears are the ones you want to avoid, and unfortunately you usually only find out the hard way.
Whether you’re thinking about something relatively straightforward like Single Storey Extensions to open the kitchen up a bit, or you’re going further with Loft Conversions to squeeze out an extra bedroom without eating into the garden, these checks don’t really change. Basement Extensions in particular are worth being extra careful with, given how much can go wrong underground in London soil, so if that’s the route you’re going, double down on the references and the insurance especially.
Final Thoughts
Honestly, most of this comes down to slowing down and asking a few uncomfortable questions before you hand over a deposit. Price matters, sure, but it’s not everything, and the cheapest quote rarely stays the cheapest once the extras start creeping in. Take your time, trust your gut when something feels off, and don’t be afraid to walk away mid-conversation if it doesn’t sit right. If you want a team that’ll actually stand up to this kind of scrutiny, London Design & Build is a solid place to start the conversation.
