Home Extension Insurance London: What You Need & When
Insurance for a home extension involves several distinct policies covering different risks at different stages. Most London homeowners only know about one of them. Here is what each policy does, when you need it, and what happens if you skip any of them.
Quick Answer
You need four types of insurance for a London home extension: (1) updated buildings insurance — notify your insurer before work starts or risk policy voidance; (2) site insurance / contract works — covers the build in progress against fire, theft and flood; (3) public liability — your contractor must carry at least £1m cover; (4) structural warranty — a 10-year latent defects policy protecting against structural failure after completion. Architect professional indemnity is the fifth layer, covering design errors.
~1% of build cost
Typical structural warranty
£1m
Min contractor PL cover
86%
Properties that skip insurer notification
Check your specific property constraints
Free Property CheckWhy Extension Insurance Is Different from Standard Home Insurance
A standard buildings insurance policy protects your home as it is today. Once you introduce a building site — scaffolding, open walls, unfinished roof sections, contractors and materials on site — the risk profile changes significantly and most standard policies are not designed for it.
Extensions, renovations, alterations and properties left partially unoccupied during works are among the most commonly excluded risks on standard home insurance policies. That exclusion is not always stated prominently. Many homeowners discover it only when they attempt to make a claim.
For a London homeowner running a rear extension or loft conversion, the financial exposure is significant: build contracts typically run from £80,000 to £200,000 and above. The combination of damage to the existing structure, theft of materials, injury to a contractor or neighbour, and post-completion structural defects represents a liability that requires specific coverage at each stage.
The five insurance layers for a London extension
Buildings Insurance: Notify Before Work Starts
Your existing buildings insurance policy must be updated before any construction work begins. This is not optional. Research consistently shows that 86% of property owners do not inform their insurer before undertaking building works — and that failure can void the policy.
What notification achieves
Notifying your insurer triggers a review of your policy in the context of the planned works. The insurer will assess the nature and value of the project and may do one of the following:
- Continue cover with no change (typically for minor works under £25,000)
- Apply an endorsement increasing the premium to reflect the construction risk
- Temporarily suspend cover for the structural works element and direct you to a specialist renovation insurer
- Decline to continue cover for the period of the works (requiring you to take out a separate site insurance policy)
Critical risk: If you fail to notify your insurer and damage occurs during the works — whether to the existing structure, the new build, or neighbouring property — your insurer may declare the policy void from the date works began and refuse any claim. This is a significant financial exposure on a project worth hundreds of thousands of pounds.
Underinsurance after completion
Even once your extension is complete, your buildings insurance needs to be updated to reflect the new rebuild cost. A rear extension adding 20–30m² to a London house increases the rebuild value significantly. According to industry data, 76% of UK buildings are currently underinsured — and the problem is worst for properties that have been extended or improved without updating the sum insured.
The sum insured on your buildings policy must reflect the full cost to demolish and rebuild the entire property to current building regulations standards, including the extension. Construction costs in the UK have risen 25–35% since 2022. Many homeowners who extended three to five years ago are now significantly underinsured unless they have actively reviewed their policy.
After your extension is signed off by building control, contact your insurer with the floor area of the new space and the contract value. Ask them to recalculate the rebuild cost and update your policy accordingly. Some insurers will commission a professional rebuild cost assessment for larger or more complex properties.
Quick Answer
Our AI can check your property's planning constraints, conservation area status, and give you a realistic cost estimate — useful context before you start talking to insurers.
£80k–£150k
Typical rear extension
£60k–£120k
Loft conversion
£70k–£140k
Side return
Check your specific property constraints
Free Property CheckSite Insurance and Contract Works Cover
Site insurance — also called contract works insurance or contractors all-risk insurance — covers the building project itself while it is under construction. This is different from buildings insurance: it protects the work in progress rather than the existing structure.
What it covers
A standard contract works policy covers the cost of reinstating or repairing the works in progress if they are damaged by:
- Fire, including accidental fire from hot works
- Flood or storm damage to unfinished roofing or open wall sections
- Theft of materials and plant from the site
- Vandalism or malicious damage to the works
- Accidental damage during the course of construction
Who takes out the policy — contractor or homeowner?
In most standard JCT Homeowner / Minor Works contracts, the contractor is responsible for maintaining contract works insurance for the duration of the project. This will be specified in the contract.
However, not all contractors carry adequate cover. Policy limits vary, and some smaller contractors carry minimal cover that would not come close to replacing a fully-framed extension destroyed by fire. Before signing a build contract, ask to see your contractor's certificate of insurance and check the sum insured against your contract value.
For larger projects, or where you cannot verify the contractor's cover, homeowners can take out their own site insurance policy for the duration of the works. Specialist providers including Hiscox, Homeprotect, Self Build Zone, and Protek offer extension-specific site insurance policies.
In London particularly, theft from construction sites is a significant risk. Materials stored overnight — steel lintels, timber, roofing materials, copper pipework — are regularly targeted. A site insurance policy covering theft of materials typically costs £300–£800 for a standard extension project and is worth having regardless of what the contractor's own policy covers.
Get a realistic cost estimate for your extension
Tell us your property address and the extension type you are planning. Our AI checks planning constraints and gives you a detailed cost breakdown including contingencies.
Public Liability Insurance: Your Contractor's Responsibility
Public liability insurance protects against claims for injury to third parties or damage to third-party property during the works. In the context of a London extension, this means covering situations like a roof tile falling and damaging a neighbour's car, scaffolding damaging a neighbouring property, or a member of the public being injured by construction activity.
What the law requires
Contractors working through a limited company are legally required to carry public liability insurance. The minimum required for most extension projects is £1 million, though most reputable London contractors carry £2 million or £5 million cover. Employers' liability insurance (covering injury to the contractor's own employees) is separately required by law and must be at least £5 million.
What to check before your contractor starts work
Important limitation: A contractor's public liability policy protects the contractor, not you as the homeowner. If your contractor causes damage to a neighbouring property and their insurer covers the claim, you benefit indirectly — but if the contractor has no valid policy or inadequate cover, you may find yourself facing a claim from your neighbour with no protection. This is why verifying cover before works start matters.
Third-party liability for homeowners
As a homeowner, your own public liability — typically included in a standard buildings or home insurance policy — may extend to cover your legal liability for injury or damage arising from your property during construction. Check with your insurer whether this remains valid during the works. If your policy is suspended or you have moved to a specialist site insurance product, ensure that your own third-party liability remains covered.
Planning an extension and need to understand the full project cost including contingencies? Our AI builds you a realistic budget.
Get a free estimate →Structural Warranties and Latent Defects Insurance
A structural warranty — also called latent defects insurance or inherent defects insurance — provides 10-year protection against structural failure in the completed extension that was caused by defects in design, materials, or workmanship.
This is distinct from buildings insurance: buildings insurance covers sudden, accidental damage. A structural warranty covers defects that were present from the time of construction but only become apparent later — a slowly failing foundation, inadequate structural steelwork, or a roof structure built to the wrong specification.
How the 10-year policy works
Most structural warranties run for 10 years from the date of practical completion and are structured in two phases:
Years 1–2: Contractor defects liability period
The contractor remains responsible for remedying any defects that emerge in the first two years. The warranty provider backs this obligation — if the contractor is insolvent or refuses to remedy the defect, the warranty insurer steps in. This is sometimes called the resolution service.
Years 3–10: Structural insurance period
From year three, the warranty policy provides direct insurance cover against major structural defects — problems with foundations, load-bearing walls, structural frames, roof structures, and waterproofing. The policyholder (you or a future buyer) claims directly against the insurer.
UK structural warranty providers
The main providers offering structural warranties for home extensions and conversions in the UK include:
- LABC Warranty — backed by Local Authority Building Control; strong in the extensions and conversions market
- NHBC Buildmark — the most widely recognised warranty in the UK; cover available for conversions and extensions up to £500,000
- Premier Guarantee — wide range of residential warranty products including extensions; £1,000 excess on claims
- Build-Zone — covers new build, conversion, renovation, and extension projects; suitable for residential and mixed-use
- Protek — self-build and extension specialist; both structural warranty and site insurance available together
- Checkmate / Global Home Warranties — competitive for extensions; used widely by architects and project managers
What a structural warranty costs
Structural warranties are typically priced at approximately 1% of the build cost for a standard extension project. On a £120,000 rear extension, that is approximately £1,200. The exact cost depends on the build value, project type, location, construction method, and — critically — when you apply. Policies must be applied for and the build inspected from the start. Retrospective warranties (applied for after completion) can cost up to 10 times more than a policy taken out from the beginning, and many providers will not offer them at all.
Key requirement: Most structural warranty providers will only issue a policy if the build has been inspected at key stages — foundations, ground floor slab, structural frame, roof structure, and weathertight stage. You cannot apply for a warranty after the extension is complete and expect the same level of cover. Budget for the warranty cost from day one and arrange it before work starts.
Why a structural warranty matters for resale
A structural warranty is transferable to subsequent buyers of the property. Mortgage lenders increasingly require evidence of a structural warranty for extensions and conversions, particularly for newer work. If you sell your London property within 10 years of completing an extension and cannot produce a structural warranty, your buyer's solicitor may flag this as a risk — potentially complicating the sale or requiring an indemnity insurance policy in lieu of the warranty.
Free AI Assistant
Ask anything about your extension project
Professional Indemnity Insurance: Your Architect's Cover
Professional indemnity (PI) insurance covers your architect or designer against claims arising from errors or omissions in their professional services — design mistakes, incorrect specifications, negligent advice, or inadequate supervision that causes you financial loss.
All registered architects in the UK are required by the Architects Registration Board (ARB) to hold adequate professional indemnity insurance as a condition of registration. The minimum level of indemnity the ARB expects is £250,000 on an each-and-every-claim basis, though most practising architects carry significantly more.
What it covers in practice
For a London homeowner running an extension project, professional indemnity insurance becomes relevant in scenarios such as:
- Incorrect structural specifications that require remedial work to the completed extension
- Planning drawings that were submitted incorrectly, causing delays or an abortive application
- Advice that the works were permitted development when they were not, resulting in enforcement action
- Failure to identify and advise on a party wall issue, leading to a dispute and legal costs
- Contract administration errors that result in overpayment to the contractor or disputed variations
What to check before appointing an architect
Structural engineers working on your extension are also required to hold professional indemnity insurance. Ask your structural engineer for the same confirmation. On a London extension project, both the architect and structural engineer are providing professional advice and design work on which you are spending significant money — their PI cover is a meaningful protection for you if their advice or work turns out to be wrong.
Insurance Timeline: What to Do and When
Most insurance mistakes on extension projects happen because actions are taken in the wrong order — typically too late. The following timeline covers the key steps and when each should happen.
Before appointing your architect
- Confirm ARB registration and PI insurance level
Before submitting planning permission
- Contact your buildings insurer to notify them of the planned works
- Discuss whether specialist site insurance will be required for the build period
Before appointing your contractor
- Request and verify public liability insurance certificate (min £2m)
- Confirm employers' liability insurance is in place
- Confirm whether the contractor's contract includes contract works insurance and at what level
- Apply for structural warranty — build inspections must start from the beginning
Once works start on site
- Ensure site insurance is active from day one
- Structural warranty inspector should attend at foundations stage
On practical completion
- Update buildings insurance to include new floor area and revised rebuild cost
- Obtain structural warranty certificate from provider
- File the warranty certificate with your property documents for future sale
Common Insurance Mistakes on London Extension Projects
Not notifying your insurer before work starts
Research shows 86% of homeowners do not inform their insurer before building works start. If damage occurs and you have not notified your insurer, they can void the policy from the date works began. On a project worth £100,000–£200,000, this is an enormous unprotected exposure.
Assuming the contractor's insurance covers everything
Your contractor's public liability policy protects them against claims — not you directly. If their cover is inadequate, has lapsed, or excludes certain works, you have no automatic fallback. Always ask to see the certificate, check the sum insured, and verify the expiry date against your programme.
Applying for a structural warranty after completion
Retrospective structural warranties exist but cost up to 10 times more than a policy applied for from the start — and many providers will simply not issue them at all if the build has not been inspected at key stages. The only way to get a standard-rate warranty is to arrange it before work begins.
Not updating the sum insured after completion
A completed extension increases your rebuild cost. With 76% of UK buildings already underinsured, and construction costs having risen significantly since 2022, leaving your buildings policy unchanged after adding a substantial extension means the sum insured may cover only a fraction of the actual rebuild value.
Not checking your architect's PI cover level
The ARB minimum of £250,000 may be insufficient for a complex London extension project where errors could require significant remedial works or generate substantial legal costs. For projects above £150,000 in contract value, asking for confirmation of PI cover above £500,000 is reasonable.
Frequently Asked Questions
Do I need to tell my buildings insurer before starting an extension in London?
Yes — you must notify your buildings insurer before any construction work begins. Failing to do so can void your policy if damage occurs during the works. Most insurers will either continue cover with an amended premium, require you to take out specialist site insurance, or suspend cover for the structural works element. The key is to notify before any works start, not after.
What is site insurance for a home extension?
Site insurance (also called contract works insurance or contractors all-risk insurance) covers the build in progress during construction against fire, flood, theft, vandalism, and accidental damage. It is different from buildings insurance, which covers the existing structure. Under a standard JCT contract, the contractor is usually responsible for taking out contract works insurance, but you should verify their cover level before works start. Specialist providers for homeowner extension projects include Hiscox, Homeprotect, Protek, and Self Build Zone.
Does my contractor need public liability insurance for my London extension?
Yes. Contractors working through a company are legally required to carry public liability insurance. For an extension project, the minimum should be £2 million, with larger projects requiring £5 million. Employers' liability insurance (minimum £5 million, required by law) must also be in place. Ask for copies of insurance certificates before signing a build contract and check the expiry dates extend to cover your full build programme.
What is a structural warranty and do I need one for an extension?
A structural warranty (also called latent defects or inherent defects insurance) provides 10-year protection against structural failure caused by defects in design, materials, or workmanship. The first two years are backed by the contractor's defects liability, with years three to ten providing direct insurance. A warranty is not legally required, but many mortgage lenders will ask for one on extensions and conversions, particularly if you sell within 10 years. It typically costs around 1% of the build cost — around £1,000–£1,500 on a standard London extension.
Which structural warranty providers cover home extensions in the UK?
The main UK providers for home extension structural warranties are LABC Warranty, NHBC Buildmark, Premier Guarantee, Build-Zone, Protek, and Checkmate (Global Home Warranties). Most offer 10-year policies. NHBC is the most widely recognised, particularly by mortgage lenders. LABC Warranty is frequently used for extensions and conversions alongside local authority building control inspection. The key is to apply and arrange inspections from the start of the build — retrospective warranties are either unavailable or cost up to 10 times more.
Does my architect need professional indemnity insurance?
Yes. All registered architects in the UK are required by the Architects Registration Board (ARB) to hold professional indemnity insurance as a condition of registration. The ARB expects a minimum of £250,000 per claim. For London extension projects, you should confirm the current PI cover level with your architect before appointing them and ensure the policy covers the full scope of services — design, planning, building regulations, and contract administration. Structural engineers working on the project should also hold PI insurance.
Summary
Running a London home extension without the right insurance at each stage is a significant financial risk. The good news is that sorting the insurance side is not complicated — it just requires action at the right points in the project timeline.
The most critical steps: notify your buildings insurer before work starts, verify your contractor's public liability cover before they set foot on site, and apply for a structural warranty before the first foundation is dug. These three actions, done in the right order, protect you against the most significant financial risks of the project.
Once the extension is complete, update your buildings policy to reflect the new rebuild cost. With 76% of UK buildings currently underinsured and construction costs having risen substantially since 2022, an extension that has not triggered a policy update is almost certainly underinsured.
Related Articles
Ready to start?
Get a free estimate and planning check for your extension
Our AI checks your property address, planning constraints, and conservation area status, then gives you a realistic cost estimate for your extension project.
Or email hello@mayfairstudio.co.uk · 07405 920944