Home Insurance · UK Research · 2026

Home insurance while an extension is being built

Standard household insurance is written for a finished, occupied home. The moment scaffolding goes up and a wall comes out, the risk changes — and almost every UK policy requires you to tell the insurer before work starts. Some insurers carry on unchanged for small jobs, some add restrictions, and some step back and expect a specialist renovation policy to sit alongside or replace the household one for the duration.

What to tell your insurer Home insurance hub

Quick answer

The essentials in 30 seconds

Tell your insurer before the first day on site. Structural work is a change in the risk you are insured for, and most policies contain a condition requiring notification of material changes.
Three outcomes are normal: cover continues unchanged for minor work, cover continues with an endorsement or restrictions, or the insurer asks you to arrange renovation or contract works cover instead.
Your builder’s insurance does not cover your house. Public liability responds to the contractor’s negligence — it is not a substitute for buildings cover on the existing structure.
Watch theft, unoccupancy and the rebuild figure. Open access, an empty house and a larger finished property are the three things that most often turn into a declined or reduced claim.

General guidance drawn from mainstream UK insurer policy documentation, published broker and industry guidance and the JCT standard contract suite. Information only — not a quote and not advice.

Declaring the works

What insurers ask, and what usually happens next

There is no single industry rule, so the honest answer to “will my policy still cover me?” is that it depends on what your insurer decides once you describe the job. What is consistent is the set of questions. Insurers want the value of the works, how long they will take, whether the structure is being opened up or supported temporarily, whether anyone will still be living there, whether scaffolding is going up, and who is doing the work. Many insurers apply an internal threshold on the value of the works — figures somewhere in the region of £25,000 to £75,000 are commonly quoted by brokers — below which minor jobs are waved through and above which the file is referred. Treat any such figure as indicative rather than a rule; the trigger in your own wording may simply be “structural alterations” with no value attached at all.

What you tell them	Typical insurer response
Cosmetic work only — decorating, new kitchen units, replacement flooring	Usually no change; most insurers do not treat this as structural
Small single-storey extension, house still occupied, low works value	Often continued with conditions noted on the file
Larger extension, structural openings, steels, roof opened up	Commonly referred — endorsement, restricted perils or excess increase
Basement dig, underpinning or substantial demolition	Frequently outside household cover; specialist renovation cover expected
House will be empty for the duration	Unoccupancy conditions apply on top of the works conditions
Scaffolding erected for several weeks	Usually accepted but noted — raises the theft and access exposure
Works being done DIY rather than by a contractor	Often the hardest to place — no contractor policy sits behind you
Works finished, extension complete	Rebuild sum insured must be increased to reflect the larger property

Indicative of mainstream UK practice in 2026. Individual insurers differ widely and some decline building works altogether. Your own schedule, endorsements and wording govern what you actually have.

Get the answer in writing, whatever it is. A phone call is fine to start, but the endorsement or confirmation email is the evidence that the insurer accepted the risk as you described it. If cover is restricted — a common pattern is theft and accidental damage being suspended for the duration, or escape of water excluded while the plumbing is disturbed — you at least know where the gap is before something happens rather than afterwards.

Who covers what

Four separate policies, and why they do not overlap

The most common misunderstanding on an extension project is that “the builder is insured” settles the question. It does not. A contractor’s public liability policy pays out when the contractor is legally liable — that is, when someone can show negligence. A fire that starts in the finished part of your house, a storm that gets into an open roof, or a theft from an unlocked site are not automatically anyone’s fault, and in those cases the only policy standing behind the building is yours.

Policy	Held by	What it is actually for
Buildings and contents (household)	You	The existing house and your possessions — subject to whatever the insurer says about the works
Public liability	The contractor	Injury or damage the contractor is legally liable for; commonly £2m of cover, often £5m or more on larger jobs
Contract works / renovation cover	You or the contractor	Damage to the works in progress, materials on site, plant and often the existing structure
Non-negligent liability (JCT clause 6.5.1)	Arranged in joint names	Damage to neighbouring property from collapse, subsidence, heave, vibration or removal of support where no negligence can be shown

Structure of cover per the JCT standard forms and published broker guidance. Not every extension needs all four — a modest single-storey rear extension rarely triggers 6.5.1, a basement dig next to a terrace almost always does.

Where a JCT contract is used, the insurance option matters. For work to an existing building — which is what an extension is — the relevant option is the one under which the existing structure and the new works are insured together in the joint names of the homeowner and the contractor. It is worth reading which option your contract has actually selected rather than assuming, because the wrong one can leave the original house outside the policy while the new brickwork is covered. Ask to see the contractor’s certificate of insurance, check the expiry date, and check that the sum insured is plausible against the value of your project.

Where claims go wrong

The four gaps that catch homeowners out

Most disputed claims during building work trace back to the same handful of issues. None of them are obscure — they are simply things that felt too minor to mention at the time.

Theft without forced entry. Household policies typically require evidence of forcible and violent entry for a theft claim. On a live site with trades coming and going, keys in a lockbox and a door off its hinges, that evidence rarely exists. Building materials on site are usually excluded from household cover entirely.
The house quietly becoming unoccupied. Moving out for “a few weeks” often turns into a few months. Once the property passes the unoccupancy period in your wording — commonly somewhere between 30 and 60 consecutive days — cover narrows sharply. Our guide on how long a home can be left unoccupied explains what typically survives and what does not.
Water and weather through an open building. A roof opened up, a wall removed or plumbing disconnected creates exactly the conditions for the most expensive routine domestic claim there is; see escape of water cover for how insurers treat it. Storm damage to an unfinished structure is frequently excluded on both household and contract works policies.
The rebuild figure left unchanged. An extension increases the reinstatement cost of the property, and the buildings sum insured is a rebuild figure, not a market value. If it is understated, an insurer may settle proportionately — reducing payment on any claim, not just one involving the new part. See buildings and contents insurance for how the two sums insured work.

Two pieces of paperwork sit alongside the insurance and are worth lining up early. In England and Wales, excavating near a boundary or working on a shared wall generally brings the Party Wall etc. Act 1996 into play, which means serving notice on the neighbours before work starts — and a party wall award is exactly the sort of document that becomes relevant if cracks appear next door. Separately, an extension needs building regulations approval and produces a completion certificate at the end. Keep it: future insurers, and any future buyer’s solicitor, will ask for it, and unapproved structural work is a recurring reason for cover being refused later.

After completion

Resetting the policy once the builders leave

Finishing the extension is a second notification, not the end of the process. The property now has more floor area, probably another bathroom or a larger kitchen, and possibly a different construction type at the back than at the front. Tell the insurer the works are complete, have the buildings sum insured reassessed to include the extension — a professional reinstatement cost assessment is the cleanest route on anything substantial — and check that any restrictions imposed for the duration have been lifted.

Two details are easy to miss. If the extension is built in a material that differs from the main house — a timber-framed or flat-roofed rear section on an otherwise brick and tile property, for instance — some insurers will treat the property as partly non-standard, which narrows the panel willing to quote; our guide to non-standard construction home insurance covers that ground. And if the new space is self-contained accommodation for a relative rather than an ordinary room, it is a different underwriting question again, dealt with in granny annexe home insurance. For where a finished, larger property sits on price, how much home insurance costs in the UK gives the broader picture, and the home insurance hub collects the rest.

Common questions

Building an extension — insurance FAQs

Do I have to tell my home insurer I am building an extension?

Yes, in practice always. Structural building work changes the risk the insurer priced, and most UK household policies carry a condition requiring you to notify a material change in circumstances during the policy term — alterations to the structure are the textbook example. Insurers also ask about planned or recent works at renewal, and answering carelessly is a misrepresentation. The consequence of staying quiet is not limited to the extension: an insurer that finds undeclared structural work can reduce or decline a claim that had nothing to do with the new building.

Will my home insurance be cancelled while the work is done?

Cancellation outright is unusual. The more common outcomes are that cover continues unchanged for minor work, continues with an endorsement that restricts certain perils for the duration, or that the insurer asks you to arrange renovation or contract works cover to sit alongside it. Insurers that will not stay on risk at all tend to say so for the heaviest jobs — underpinning, basement excavation, substantial demolition — or where the property will be empty throughout. Ask what the position will be before you commit to a start date, since finding out mid-project is far harder to fix.

Does my builder’s insurance cover damage to my house?

Only where the builder is legally liable. Public liability insurance — commonly £2m of cover, often more on larger contracts — responds when the contractor’s negligence causes injury or damage. It does not respond to a storm getting into an open roof, a fire with no identifiable cause, or a theft from an open site. It also will not rebuild your existing house. Ask to see the certificate, note the limit and the expiry date, and treat it as protection against the builder’s mistakes rather than as cover for your property.

What is renovation or contract works insurance?

It is a policy written for a property that is being altered rather than lived in unchanged. A typical package covers the works in progress and materials on site, extends buildings and contents cover to the existing structure on terms that account for the disturbance, handles the property being unoccupied, and can include public liability and the non-negligent liability cover a JCT contract may require. It is normally arranged for the length of the project plus a margin, and some policies can be extended if the job overruns — worth checking at the outset, because overruns are the norm.

Am I covered if tools or materials are stolen from the site?

Usually not under household insurance. Building materials awaiting installation are typically excluded from a standard buildings and contents policy, and the contractor’s own tools are the contractor’s responsibility. Theft claims for your own possessions can also fail during works because most policies require evidence of forcible and violent entry, which is hard to demonstrate on a site with trades holding keys and openings in the structure. Contract works cover is the policy designed for materials on site; scaffolding, which gives access to upper floors, is one of the things insurers specifically ask about.

What if we move out while the extension is built?

Then two sets of conditions apply at once — the works conditions and the unoccupancy conditions. Most household wordings restrict cover after the property has been unoccupied for a set number of consecutive days, commonly somewhere between 30 and 60, typically cutting back to a limited group of perils and adding requirements such as regular inspections and draining the water system in winter. Tell the insurer the property will be empty at the same time as you declare the works, not later, and be realistic about the duration.

Do I need JCT clause 6.5.1 non-negligent liability cover?

Only if your building contract calls for it, and mainly where the work could affect a neighbouring property. Clause 6.5.1 deals with damage to other people’s property caused by collapse, subsidence, heave, vibration or the weakening or removal of support arising from the works, in situations where nobody has been negligent — a gap that ordinary public liability leaves open. It is arranged in the joint names of the homeowner and the contractor. Basement digs, underpinning and excavation close to a party wall are the classic triggers; a modest garden-side extension usually is not.

How much will the extension change my premium?

There are two separate movements and it is worth keeping them apart. During the works, any change reflects the temporary risk, and it may take the form of a higher excess or restricted perils rather than a higher premium. After completion, the property is permanently larger with a higher rebuild cost, more rooms and often another bathroom or kitchen, and the ongoing premium generally rises to match. We do not publish figures for this because the range across insurers, projects and construction types is too wide to be meaningful — the reliable step is an updated reinstatement cost assessment and a fresh set of quotes at renewal.

Do I need to tell my insurer about planning permission or building regulations?

Insurers do not police planning, but they do care whether structural work was properly approved. An extension needs building regulations approval whether or not planning permission was required, and the completion certificate at the end is the document that proves it. Keep it with the deeds. Unapproved or undocumented structural alterations are a recurring reason for cover being refused or restricted years later, and a buyer’s solicitor will ask for the same paperwork when you sell. In England and Wales the Party Wall etc. Act 1996 is a separate obligation to the neighbours, not to the insurer, but the award it produces is useful evidence if damage is alleged next door.

Information only — not financial advice. Figures and positions described are indicative and for orientation, not quotes; policy terms, limits, conditions and exclusions vary by insurer and are set out in each policy’s own wording and schedule. Contract and party wall requirements described reflect the JCT standard forms and the law of England and Wales; Scotland and Northern Ireland differ. My Insurance Expert is not an FCA-authorised intermediary and does not arrange or sell policies. Last updated: 2026-07-21

Explore more

More Home Insurance pages

← Back to Home insurance