Extensions go wrong for a handful of predictable reasons: a builder taken on without proper checks, a quote that was too vague to compare against anything, or a payment schedule that left the homeowner exposed. None of that is bad luck, it is avoidable with the right process before you sign anything.
Before you start vetting anyone, it helps to know roughly what you are budgeting for. MyJobQuote’s single storey extension cost guide puts a typical UK rear extension at £25,000 to £40,000, or around £1,800 per square metre, with basic finishes closer to £1,000 to £1,700 per square metre and a high spec finish running £1,900 to £2,200 per square metre. A builder’s quote should make sense against that range, not sit wildly outside it in either direction.
Where to actually look
Word of mouth is a genuinely strong signal, but only if you can see the finished work and it was a comparable job, an extension is a different scale of project to a bathroom refit. Beyond personal recommendations, look at:
- The Federation of Master Builders (FMB) directory, which vets members against trading history and standards before admitting them.
- Independent review platforms, reading the actual content of reviews rather than just the star average. A run of reviews mentioning the same specific strengths (tidy site, kept to timeline, good communication) is more useful than a high average built on a handful of ratings.
- Companies House, if the builder trades as a limited company, to check how long they have actually been trading. A business that has existed for years under the same name is a meaningfully lower risk than one a few months old.
This is the same process we use to build our Top 5 lists of builders for every town we cover, so a page for your area is a shortcut through this exact vetting process.
Red flags to walk away from
- No fixed address or a mobile number only, with no way to verify a business actually exists beyond a phone call.
- Pressure to decide quickly, or a price that is only valid “if you sign today.” Genuine tradespeople with a full diary are not desperate for your job specifically.
- Reluctance to provide references or show previous work. Anyone doing consistently good work wants to show it off.
- A quote with no detail, just a single total figure with no breakdown of materials, labour, and what is and is not included.
- Insisting on cash only, with no invoice. This is a red flag for accountability if a dispute arises later, not just a tax question.
What a proper quote should include
A quote you can actually compare against others should specify: the exact scope of work, materials by type and quality (not just “brick” but which brick, not just “windows” but what specification), who is responsible for building regulations applications and inspections, an estimated timeline with key milestones, and a payment schedule tied to stages of completion rather than a single upfront sum. If a builder cannot produce this level of detail, it usually means they have not properly costed the job, which is its own warning sign.
Planning permission and building regulations
Many single storey rear extensions fall under permitted development and do not need full planning permission, but the rules depend on the size, your property type, and whether you are in a conservation area, so never assume. The Planning Portal, the official government-backed planning service, is the place to check before you commit to a design. Building regulations approval is a separate matter and applies to almost every extension regardless of planning permission, covering things like structural safety, insulation and fire escape. A reputable builder will tell you clearly which of these apply to your job, and who is responsible for submitting for them.
Get it in writing before work starts
Even with a trusted builder, a written contract covering scope, price, timeline, payment stages, and what happens if either side wants to change something partway through protects everyone. It is not a sign of distrust to ask for this, it is standard practice, and any reputable builder will expect it.
Frequently asked questions
Should I use a builder recommended by a friend, or find my own?
A recommendation from someone who used the same builder for a similar sized job is genuinely valuable, since it is evidence you can trust more than a review left online. It should still be one input rather than the whole decision. Ask to see the finished work in person if possible, and check the builder independently in the same way you would any other candidate.
How many quotes should I get for an extension?
Three is the usual sweet spot. Fewer than that and you cannot tell if a price is reasonable. More than three tends to add time without adding much useful information, and some builders are reluctant to quote seriously if they suspect they are one of six competing bids.
Is it normal to pay a deposit before work starts?
A reasonable deposit, often to cover the cost of materials ordered up front, is normal. What is not normal is a large deposit relative to the total job value, or being asked to pay in cash with no invoice. A staged payment schedule tied to completed work, rather than one big payment upfront, protects you if something goes wrong partway through.
Do I need planning permission or building regulations approval for an extension?
It depends on the size and type of extension. Many single storey rear extensions fall under permitted development and do not need planning permission, but almost all extensions need building regulations sign off regardless. A reputable builder will tell you clearly which applies to your job rather than leaving you to find out.
About the author
Stu Clark
Stu spent over 15 years in the trades, working as a plumber before running his own plumbing and bathroom renovation company with a team of eight. He founded Top5Trades to give homeowners the kind of shortlist and straight advice you would get from someone who has actually done the work. More about us →