Can you get a building warranty for an extension or renovation?

The answer to whether you can get a builder’s warranty for an extension is yes, you can. But, for most home extensions, it won’t look like the 10-year structural warranty that comes with a new-build house. 

What your builder offers is usually a written workmanship guarantee, and its real value depends on whether an insurance backed guarantee (IBG) sits behind it. A full structural warranty tends to come into play when a project creates a new dwelling, such as a barn conversion, rather than adding to an existing home. Protection while the work is under way is a separate question again. 

Below, we set out what each type of cover does, which one fits your project, and what to ask your builder before anything is signed.

What a Builder’s Warranty for an Extension Usually Means

The term gets used loosely, and it can refer to four quite different things:

• Builder’s workmanship guarantee: a written promise from your builder to put right defects in their work for a period set out in your contract. It’s only as good as the business behind it, so if the builder stops trading, the guarantee usually goes with them.
• Insurance backed guarantee (IBG): an insurance policy that honours the builder’s guarantee if they cease trading. Builders registered with TrustMark must provide a minimum two-year IBG on domestic work, and longer terms are available.
• Structural warranty: a 10-year insurance policy against major defects in the structure, such as the foundations, load-bearing walls and roof. It’s standard on new builds and conversions, where mortgage lenders expect one.
• Building control completion certificate: confirms the work met Building Regulations when it was signed off. It isn’t a warranty and won’t pay for defects that appear later.

For more on what each of these typically includes, see our guide to what a builder’s warranty covers.

Which Cover Fits Your Project?

The right cover depends less on the size of the job and more on what it creates. Renovations can be done to upgrade any type of existing structure, whereas conversions tend to be changing it more extensively from one purpose to another. Extensions tend to be smaller projects, which mainly involve adding a new part onto the main building. 

• Extensions, loft conversions and basement conversions: the work adds to an existing home, so a builder’s guarantee backed by an IBG is the usual long-term protection. Mortgage lenders don’t generally ask for a structural warranty on this kind of work. A buyer’s conveyancer will look for building control sign-off instead.
• Renovations and refurbishments: the same applies. Structural alterations, such as removing a load-bearing wall, still need Building Regulations approval, and your home insurer needs to know before work starts.
• Conversions that create a new home: turning a barn, church, shop or farm building into a house or flats changes the picture. The UK Finance Mortgage Lenders’ Handbook states that lenders will generally only lend on newly built, converted or renovated property covered by a warranty scheme or a Professional Consultants’ Certificate, so cover needs arranging before work begins. See our barn conversion warranty and Professional Consultants’ Certificates.
• Projects you manage yourself: if you’re acting as your own project manager on a new home or conversion, a self-build structural warranty is designed around that route.

Protecting Your Home While the Work Is Under Way

When carrying out renovation or extension works, you might think that you can simply rely on your builder or contractor’s insurance to cover the project. However, though the builder will have a contract with you, their insurance policies will be established to safeguard their interests, not necessarily yours. 

For example, they’re likely to have the following policies set up as a business:

• Public liability insurance: covers third party accidents or injuries on the worksite
• Employers’ liability insurance: covers any workers employed by the builder/contractor
• Contract works (site) insurance: covers the new works and materials on site during the build. Whether it extends to your existing house depends on the contract.
• Legal expenses insurance: covers the builder/contractor in case of a contractual dispute

These policies essentially cover the workers and their work while onsite for the duration of the project – , but not your investment as the property owner, and not before and after. Even in the case of liability insurance, you would have to prove their negligence was at fault in the case of injury or property damage. 

The existing structure is usually yours to insure. Where your project uses a JCT contract, its insurance options set out who insures the new works and who insures the existing building. For works to an existing home, the owner is often responsible for keeping the existing structure insured, so read that clause with your builder before work starts. For larger projects, and for self-builders running their own site, dedicated contractors all risk insurance protects the works, materials and plant through to completion.

Tell your home insurer before work begins. Even if you already have home and contents insurance and a warranty in place for yourself, you can’t assume that they’ll cover the new construction and still apply when the work is completed. Many home insurance policies restrict cover while building work is under way, or while a property is left unoccupied during the build. If the work isn’t declared and something goes wrong, the insurer can decline the claim because your circumstances have changed.

If your home is still within the term of a structural warranty, speak to the warranty provider as well. Alterations to the structure can affect cover on the parts of the building they touch.

Damage to neighbouring property. Excavation, underpinning or structural work close to a boundary can damage a neighbour’s property even when nobody has been negligent, and a builder’s public liability cover typically only responds where negligence is proven. JCT non-negligence cover, often referred to as JCT 6.5.1, is designed for that gap. 

If the work affects a shared wall or involves digging foundations near a neighbour’s building, you may also need to serve notice under the Party Wall etc. Act 1996.

Protecting the Work After Completion

How an IBG works in practice. An IBG is usually registered before the work starts and issued once it’s complete. It covers defects that fall within the builder’s written guarantee if the builder is no longer trading to put them right. 

Some policies also protect deposits and stage payments during the build, while others only start on completion, so check which type you’re being offered and ask for proof the project has been registered before handing over a deposit. ABC+ Warranty provides insurance backed guarantees alongside a builder’s workmanship guarantee.

When a structural warranty is relevant. A structural warranty relies on a surveyor inspecting the build in stages, from foundations through to completion, so it normally needs to be in place before work begins. For a conversion creating a new dwelling, arranging it at the start avoids the more involved retrospective route later.

Selling a home with an extension. For a standard extension, a buyer’s conveyancer will usually ask for building control sign-off, planning permission where it was needed, and copies of any guarantees for the work. A structural warranty isn’t normally on that list, but a guarantee that can pass to a new owner gives buyers reassurance that the work is still covered. 

Our guide to whether building warranties are transferable explains how cover moves between owners, and our article on building regulations approval for renovations covers the sign-off side.

What If the Work Is Already Finished?

If a conversion or new dwelling was completed without a warranty, cover can still be possible. A post-completion structural warranty, such as our Completed House Warranty, involves a single detailed survey of the finished building, and a retrospective PCC may suit some cases. 

Our guide to how a post-completion building warranty works explains the process. For a standard extension finished without an IBG, the options are narrower, as IBGs usually need registering before the work begins.

Questions to Ask Your Builder Before Work Starts

A few questions at quote stage remove most of the uncertainty later. Get the answers in writing, ideally as part of the contract:

• How long is your written guarantee, and what exactly does it cover?
• Is it backed by an insurance backed guarantee, and who provides it?
• Does the IBG protect deposits and stage payments, or only defects after completion?
• Can I see your public liability and contract works insurance certificates?
• Under our contract, who insures the existing house during the works?
• Who is applying for building control approval, and will I receive the completion certificate?
• Does the work need Party Wall notices, and who is serving them?

Why Choose ABC+ Warranty

ABC+ Warranty policies are backed by AXA Insurance, and our team includes surveyors and building control specialists with RICS, CIOB and CIAT backgrounds. Our construction and property insurance is arranged by Nicola Smith, a CII-qualified broker with almost 20 years’ experience in the construction and property sectors.

• Insurance backed guarantees alongside a builder’s workmanship guarantee
• Structural warranties for new builds, conversions and self-builds
• Professional Consultants’ Certificates where a PCC suits the project
• Construction insurance for contractors and self-builders, including JCT non-negligence cover

Because we arrange all of these, we can match the cover to your project rather than fitting your project to a single product. You can check which lenders accept our cover on our Warranty Lender List, or read more about our structural warranties.

Talk to us about your extension, renovation or conversion: call 0161 928 8804, message us on WhatsApp, or email abc@architectscertificate.co.uk.

FAQs

Does building control sign-off count as a warranty?

No. A completion certificate confirms the work met Building Regulations when it was inspected. It doesn’t cover defects that appear later, and it doesn’t protect you if the builder stops trading.

Do I need a structural warranty to sell a house with an extension?

Not usually. A buyer’s conveyancer will typically look for building control sign-off, any planning consent and guarantees for the work. A structural warranty or PCC becomes relevant where the property itself is a new build or recent conversion, as set out in the UK Finance Mortgage Lenders’ Handbook.

Does an IBG protect my deposit and stage payments?

It depends on the policy. Some IBGs include deposit and stage payment protection during the build, while others only cover defects after completion. Ask your builder which type they’re offering before you pay anything.

Will my home insurance cover the house while the extension is being built?

Not automatically. Many policies restrict cover during building work or while a property is unoccupied, so tell your insurer before work begins and get their confirmation in writing.

How long does a builder’s guarantee on an extension last?

It varies by builder and should be stated in your contract. TrustMark-registered builders must back their work with an IBG of at least two years, and longer terms are available: the FMB Building Work Guarantee, for example, runs for 2, 6 or 10 years.