Ask most builders if they're insured and you'll get a yes, a nod, and no paperwork. The problem is what that yes actually means: your builder's insurance exists to protect your builder. Whether anything protects you, your house, and your half-built extension is a separate question, and you need to answer it before anyone breaks ground.
Why "Yes, I'm Insured" Is Not the Same as Being Protected
Public liability insurance, the policy most builders mean when they say they're insured, only responds when the builder is legally liable for damage or injury. That means negligence. If a storm rips the membrane off your half-built roof overnight, nobody was negligent, so the builder's policy doesn't respond. Neither does yours, if your insurer doesn't know the works are happening. A structure worth tens of thousands of pounds can sit completely uninsured while everyone involved believes they're covered.
That is the exposure gap. Closing it takes three checks on the builder's side, plus one phone call and one decision on yours.
The Three Covers to Verify
Public liability
Covers injury to third parties and damage to property, yours or next door's, caused by the builder's negligence. A digger through the neighbour's drains, a scaffold pole through a conservatory roof: this policy pays.
It is not a legal requirement at any level, on any project. That surprises people. Public liability is a market expectation, not a statutory duty, so the only thing forcing your builder to hold it is you. £2m is the common floor on domestic work; on structural work like an extension, £5m is the level to push for. A builder's own liability premium runs roughly £100-£800 a year depending on trade and cover level, so "insurance is too expensive" is not an explanation. It's a warning.
Employers' liability (the legal one)
If your builder employs anyone who isn't close family, including labour-only subcontractors, employers' liability insurance is compulsory under the Employers' Liability (Compulsory Insurance) Act 1969 and the 1998 Regulations made under it, at a minimum of £5 million. HSE can fine an uninsured employer up to £2,500 for every day without cover, plus £1,000 for failing to produce the certificate when asked. That regime covers England, Wales and Scotland; Northern Ireland has its own parallel legislation with the same daily penalty.
Why should you care about the builder's legal duty? Because a labourer injured on your land with no employers' liability policy behind him will look for someone else to claim against, and the person who owns the site is the obvious candidate.
Contract works (the one nobody thinks about)
Also sold as "contractors' all risks". This covers the new works themselves, the half-built structure and the materials sitting on site, against fire, storm, flood and theft. Neither liability policy touches any of that, because nobody has to be negligent for a storm to arrive.
On an extension to an occupied house, never assume the builder carries this. Ask directly, and ask what the policy excludes. Some tradesperson policies specifically exclude fire caused by "application of heat", which means blowtorches and grinders, which means exactly the work that happens on extensions.
One thing no policy on this list pays for: defective workmanship. Bad blockwork is a contract, retention and warranty problem, never an insurance claim.
| What goes wrong | Which policy responds |
|---|---|
| Builder's negligence damages your house or a neighbour's | Builder's public liability |
| A worker is injured on site | Builder's employers' liability |
| Fire, storm or theft hits the half-built works | Contract works insurance, if anyone holds it |
| Storm damages your existing house through no one's fault | Your own buildings insurance, if you've told them about the works |
| Poor workmanship | None of them. That's contract and retention territory |
How to Check Builders Insurance Is Genuine
Ask for the certificates for all three covers before you sign anything, then run five checks:
- Match the named insured to your contract. If you're contracting with "J Smith Building Ltd" and the certificate names "John Smith, sole trader", the policy may not respond to your job at all. The name on the certificate must match the entity on your contract.
- Check the dates against your build programme. A policy expiring in month two of a five-month build is a diary item, not a reassurance. Ask how renewal will be evidenced.
- Check the cover limit against the damage a bad day could do. Think contract value plus the rebuild value at risk. £1m of public liability on a job attached to a £400,000 house is thin.
- Phone the insurer or broker. Use contact details you find yourself, not the ones printed on the certificate. Quote the policy number and ask whether the policy is in force.
- Treat refusal as your answer. A builder who won't produce certificates has told you everything you need to know. Walk.
Be realistic about the limits of all this. Certificates can be forged, some insurers won't confirm policy details to a third party, and a certificate only proves cover existed on the day it was issued. No verification method available to a homeowner is 100% foolproof. The five checks above are the strongest realistic filter, and the builders they filter out are precisely the ones you wanted filtered.
Tip
Keep a copy of every certificate together with the date you verified it. If a claim ever surfaces, evidence that you checked cover before work started strengthens your position with your own insurer and in any dispute.
Whatever you verify, write it down. Your contract should specify the covers the builder must hold, the minimum limits, and the obligation to keep them in force for the whole job. Our guide to contracts and payment schedules shows where those clauses sit alongside the payment terms, and if you're agreeing the deposit at the same time, the same paperwork discipline applies: see builder deposits and stage payments.
Your Own Exposure: The Checks Nobody Does
Warning
Tell your buildings insurer before work starts. Standard home insurance policies routinely exclude or restrict cover during structural works, and failing to notify can void the entire policy, not just claims connected to the build. Some insurers simply note the works. Some add a modest premium or excess. Some decline once the project value crosses their threshold, with £50,000 a cutoff one insurer has applied. Have that conversation before the digger arrives, not after the claim.
If your insurer restricts cover, or the builder's contract works position is weak, specialist site or renovation insurance fills the gap. It covers the existing structure and the new works together, under one policy, in your name.
0.5-1%
Specialist site insurance is priced as a percentage of the total build cost. On a £100,000 extension that's £500-£1,000, which reads differently once you picture the alternative: a fire-damaged shell with no payout behind it.
If the plan is to rely on the builder's contract works policy instead, ask one more question: are you a joint named insured, or merely "noted" on the policy? Joint names means you can claim directly, you're told if the policy lapses or is cancelled, and the payout can follow the project to a replacement contractor. Being noted means the payout goes to the builder, who is under no obligation to spend it rebuilding your extension. Ask which arrangement you're getting, and get the answer in writing.
The insurance and liability guide works through this decision step by step, including the exact questions to put to your insurer and your builder before the start date.
Explore the full Kitchen Extension Guide
Every task, in the right order, with tools and materials guidance.
View the GuideWhat Happens If the Builder Is Uninsured and Something Goes Wrong
The costs don't disappear. They look for the nearest solvent person, and that is usually you.
A worker injured on your land can bring a claim against you as the occupier, and even a claim you successfully defend costs money and months. A cracked party wall or a flooded neighbouring kitchen becomes your problem to resolve, because your neighbour lives next to you, not next to the builder. And a half-built extension lost to fire or storm with no contract works cover is simply gone: the stage payments you made for that work are spent, and rebuilding it comes out of your pocket a second time.
Suing an uninsured builder rarely fixes any of this. A county court judgment against an individual with no insurer and no assets is a piece of paper. Enforcement is slow, recovery rates are poor, and plenty of homeowners win the case and never see a pound. Verification before work starts is cheap. Recovery after the event frequently isn't possible at any price.
A Note on the Contract and CDM 2015
Under the Construction (Design and Management) Regulations 2015, health and safety duties on a domestic project transfer automatically to the contractor. Commissioning an extension does not make you a site manager. You do keep a residual duty: appoint competent people, and allow enough time and money for the work to be done safely. Checking insurance certificates, references and trade body membership is a large part of how a homeowner demonstrates "competent people" in practice.
What a Real Build Looked Like
On a real 30m² kitchen extension in Oxfordshire (£172,900 all-in, £120,480 of construction cost excluding the kitchen itself), the builder supplied his insurance certificate unprompted, three days after the deposit was paid and one day before the contract was signed. That is what good practice looks like: paperwork offered, not extracted under pressure.
The same build shows why insurance is the backstop rather than the everyday tool. Mid-build, a gas leak connected to the works cost £856.67 to put right. No insurance claim was ever made. Instead, £150 was deducted from the builder's final invoice, negotiated and documented as part of the final settlement. Day-to-day incidents get resolved through the contract and the payment schedule, because that route is faster and keeps the relationship working. The insurance sits behind it as the answer to a different question: what if this had been a £20,000 problem instead of an £856.67 one?
And insurance did nothing to stop the same builder quietly substituting a timber ridge beam where the structural design specified steel, a change nobody was told about and that was caught only when Building Control inspected. No policy prevents bad decisions on site. A written contract, stage payments you control, and staying engaged with the work do that job. Verifying insurance is one afternoon within the longer discipline of managing your builder relationship across the whole programme.
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Before They Start
Ten minutes of paperwork before the contract is signed: three certificates, names matched, dates checked, limits sane, one phone call to the insurer, and one call to your own. That is the whole discipline. Almost nobody does it, which is exactly why the horror stories keep being written.
Insurance is one task in the pre-construction phase, alongside contracts, payment schedules, party wall notices and building control. The free kitchen extension tree shows every task in order, and the full Kitchen Extension Access Pass gives you access to every task guide for £49.
Frequently Asked Questions
What insurance should a builder have for an extension?
Three covers: public liability (£2m minimum, £5m preferred on structural work), employers' liability (£5m minimum, legally required if they have staff or labour-only subcontractors), and contract works insurance protecting the build itself against fire, storm, flood and theft. Ask to see all three certificates before you sign the contract.
Is builder's insurance a legal requirement?
Only employers' liability, and only if the builder employs staff or labour-only subcontractors who aren't close family. The 1969 Act and its 1998 Regulations set the £5m minimum, with HSE fines of up to £2,500 per day for going without. Public liability and contract works cover are never legally required, which is exactly why your contract needs to require them instead.
Do I need my own site insurance for an extension?
Often, yes. Standard buildings insurance frequently excludes or restricts cover during structural work, and the builder's public liability only responds to his negligence, not to storms, fires from unrelated causes, or theft. Notify your insurer before work starts, and if they won't extend cover, specialist renovation insurance at 0.5-1% of the build cost fills the gap.
What happens if my builder is uninsured?
The costs look for the nearest solvent person, which is usually you. An injured worker can claim against you as the occupier, a neighbour's damage claim lands on your doorstep, and a half-built extension lost to fire has no payout behind it. Court judgments against uninsured individuals are hard to enforce and often recover nothing, so verify cover before work starts, not after an incident.