Introduction
Many homeowners assume that as long as they have a home insurance policy in place, they're properly protected. In reality, the amount you're insured for matters just as much as having a policy at all, and an inaccurate sum insured can significantly reduce what you actually receive when you make a claim, even a claim that has nothing to do with the reason your figure was wrong in the first place.
This guide expands on the sum insured points touched on in our main Home Insurance UK article, focusing specifically on how underinsurance happens, what it actually costs you at claim time, and how to avoid it using genuine rebuild cost data rather than guesswork.
Key Terms Explained
- Sum Insured
- The maximum amount your policy is set up to pay out, which you choose and which should reflect the true rebuild cost of your property or the true value of your contents.
- Average Clause (Condition of Average)
- A policy term allowing an insurer to reduce a claim payout proportionally where the sum insured is lower than the actual value at risk, applied to partial claims as well as total losses.
- Rebuild Cost
- The cost to demolish and completely rebuild a property from scratch, including materials, labour, professional fees and site clearance, which is typically very different from its market or sale value.
- BCIS House Rebuilding Cost Index
- An index calculated by the Building Cost Information Service (BCIS) on behalf of the Association of British Insurers (ABI), used across the UK insurance industry to index-link buildings sums insured between full reviews.
- Indemnity vs New-for-Old Cover
- Indemnity cover pays out the depreciated value of damaged or lost contents, while new-for-old (reinstatement) cover pays the cost of a genuinely equivalent replacement, a distinction that becomes more significant the more underinsured a policy is.
What Is Underinsurance?
Underinsurance occurs when the sum insured you've selected on your policy is lower than the true value of what you're insuring, whether that's the cost to rebuild your property or the total value of your contents.
It's More Common Than Many People Realise
Underinsurance is a widespread issue precisely because it often goes unnoticed until a claim is made. A policy can run for years with an inaccurate sum insured without any obvious problem, until a significant loss reveals the shortfall. Industry commentary reported across UK insurance and broker publications during 2025 and 2026, drawing on broker surveys and property assessment data, has repeatedly highlighted underinsurance as a widespread problem across UK residential property, with the majority of assessed properties found to be insured for less than their true rebuild cost. These figures come from private broker and assessment firm surveys rather than an official government or regulator statistic, so they should be read as an industry-reported trend rather than a precise national figure, but the consistent direction, that most UK homes carry some degree of underinsurance, is echoed across multiple independent sources.
Why It Matters So Much
The consequences of underinsurance aren't simply a smaller safety margin, they can directly and proportionally reduce what you're paid on any claim, not just a total loss, which is a detail many homeowners are unaware of.
The Average Clause Explained
Many UK home insurance policies include what's known as the average clause, also called the condition of average, which allows the insurer to reduce a claim payout if the property or contents were underinsured at the time of the loss.
How the Reduction Works
In simple terms, if your sum insured only reflects a percentage of the true value at risk, the average clause can reduce your claim payout by that same percentage, even if the claim itself is only for partial damage rather than a total loss.
A Worked Example
Suppose your home would genuinely cost £300,000 to rebuild, but your policy's sum insured is set at £210,000, which is 70% of the true figure. If a kitchen fire then causes £20,000 of damage, a straightforward application of the average clause would reduce your payout to roughly 70% of that claim, around £14,000, leaving you personally responsible for the remaining £6,000, despite the damage itself having nothing to do with the underinsurance.
Why Partial Claims Are Still Affected
A common misconception is that underinsurance only matters for a total loss. In fact, the average clause typically applies to partial claims too, meaning a relatively minor claim can still be reduced if the underlying sum insured was too low.
Not All Policies Include It
Not every insurer applies an average clause in the same way, and some policies have more favourable terms for minor discrepancies. Checking your policy wording, or asking your insurer directly, clarifies exactly how underinsurance would affect your specific policy.
Buildings Sum Insured: Rebuild Cost, Not Market Value
One of the most common sources of underinsurance is confusing a property's market value with its rebuild cost, which are often very different figures.
What Rebuild Cost Actually Means
Rebuild cost reflects what it would cost to demolish and completely rebuild your home from scratch, including materials, labour, professional fees and site clearance, rather than what the property might sell for on the open market.
Why the Two Figures Diverge
Market value is heavily influenced by location, land value and local demand, while rebuild cost depends on construction type, size, materials and current building costs. In many parts of the UK, especially high-demand areas, market value can be considerably higher than rebuild cost, and in others, particularly for larger or older properties, rebuild cost can exceed market value.
Estimating Rebuild Cost Accurately
The ABI's own rebuilding cost calculator, built by BCIS, is designed to give a broad guide for UK properties of relatively straightforward construction, typically standard two-storey homes built from brick or stone, adjusted for regional labour and access costs and the property's specification. It's intended to support, not replace, professional advice, and its own guidance flags that unusual features, thatched buildings, listed status, or non-standard materials fall outside what the calculator can reliably estimate. For larger, older, listed or non-standard properties, a professional surveyor's rebuild cost assessment is often worth the investment for accuracy.
A Mortgage Balance Is Not a Rebuild Cost
A genuinely common point of confusion is between the outstanding mortgage balance and the buildings rebuild cost. A mortgage lender will usually require buildings insurance as a condition of the loan, and some borrowers assume the sum insured simply needs to match, or exceed, what's still owed on the mortgage. These are entirely unrelated figures: your mortgage balance reflects what you owe the lender, while rebuild cost reflects what it would actually cost to reconstruct the property. A property with a small remaining mortgage balance can easily have a rebuild cost many times higher, and insuring only to the mortgage balance, rather than the true rebuild cost, is itself a direct route into underinsurance.
Why Rebuild Costs Keep Moving
A sum insured isn't a figure you set once and forget. Rebuild costs move over time, sometimes sharply, driven by changes in the price of materials, labour and site access.
What the Industry's Own Data Shows
The BCIS House Rebuilding Cost Index, calculated for the ABI and widely used across the UK insurance industry for index-linking buildings sums insured between full reviews, recorded year-on-year rebuild cost increases running into double digits through much of 2021, 2022 and into 2023, reflecting a period of exceptional materials and labour cost inflation across UK construction. Annual increases then moderated significantly from 2024 onwards, settling into the low single digits, though rebuild costs have continued rising every year across this entire period, never falling.
Why This Matters for a Sum Insured Set Years Ago
If your buildings sum insured was last properly reviewed before this period of elevated inflation, and simply auto-renewed each year without adjustment, there's a real possibility it now understates your true rebuild cost by a meaningful margin, even if nothing about the property itself has changed. BCIS's own guidance recommends checking rebuild cost directly at least every five years, rather than relying on annual index-linking alone over a long period, precisely because an index reflects an average across many house types and can't exactly track every individual property's actual rebuild cost.
Index-Linking Helps, But Isn't a Substitute for a Proper Review
Many buildings insurance policies apply automatic index-linking each year to keep the sum insured broadly in step with rebuild cost inflation. This reduces drift but doesn't correct an inaccurate starting figure, and it can't account for property-specific changes such as an extension or renovation that a general index was never designed to capture.
Contents Underinsurance
Underinsurance isn't limited to buildings cover. Contents insurance is just as vulnerable, often for different reasons.
Underestimating Cumulative Value
Many people underestimate contents value because they think item by item rather than adding up the true total. Furniture, electronics, clothing, kitchen equipment and everyday possessions add up to a surprisingly large figure when properly totalled.
Forgetting High-Value or Less Obvious Items
Items such as jewellery, technology, and even accumulated smaller possessions can be overlooked when estimating contents value, leading to a sum insured that doesn't reflect what would actually need replacing after a total loss such as a fire.
Not Updating Cover After Purchases
Contents value tends to increase over time as new items are bought, yet many policyholders never revisit their contents sum insured after initially setting up a policy, gradually drifting into underinsurance.
For cover on valuable items you take outside the home, see our Personal Possessions Insurance UK guide.
Common Causes of Underinsurance
Choosing a Lower Sum Insured to Reduce Premiums
Some homeowners deliberately understate their sum insured to lower premiums, not realising that this can significantly reduce claim payouts through the average clause, ultimately proving a false economy.
Renovations and Extensions Not Reflected in Cover
Extending or significantly renovating a property increases its rebuild cost, but this is easy to overlook when reviewing insurance, particularly if the policy simply auto-renews each year without adjustment.
Inflation in Building and Material Costs
As BCIS's own index confirms, construction costs can rise considerably over a relatively short period, and a sum insured that was accurate several years ago may no longer reflect current rebuild costs if it hasn't been reviewed and adjusted since.
Relying on Outdated Valuations
Using an old valuation, whether for buildings or contents, without checking whether it still reflects current costs and possessions is a common and easily avoidable cause of underinsurance.
Carrying Over a Figure From a Previous Policy or Owner
Some buyers simply match the sum insured shown on a previous owner's policy documents, or an outgoing insurer's renewal figure, assuming it must be correct. That figure may itself have been inaccurate, or may simply be out of date by the time you take on the property, so it should be treated as a starting point to verify, not a figure to copy directly.
Whose Responsibility Is It to Get This Right?
It's a reasonable question, and worth answering honestly: under most UK home insurance policies, setting an accurate sum insured is fundamentally the policyholder's responsibility, not the insurer's. An insurer will generally ask you to state a figure, sometimes with the help of a calculator or questions about your property, but it doesn't independently verify your rebuild cost or contents value before accepting your premium.
What Insurers Are Expected to Do
The FCA's Consumer Duty, which applies across UK financial services including insurance, requires firms to support customers in making effective, informed decisions and to avoid causing foreseeable harm. In practice, this means insurers should present sum insured questions clearly and avoid designs that make significant underinsurance more likely, but it does not shift the underlying responsibility for the figure itself away from the policyholder.
Why This Makes a Genuine Review Even More Important
Because the onus sits primarily with you, an insurer generally won't proactively flag that your sum insured looks low until a claim is actually made, by which point the average clause may already apply. This is precisely why building a habit of periodically checking your rebuild cost, rather than assuming your insurer would step in if something were wrong, matters so much.
How to Set an Accurate Sum Insured
Use Dedicated Rebuild Cost Tools
Rebuild cost calculators designed specifically for insurance purposes, such as the ABI/BCIS calculator, account for construction type, floor area, location and other relevant factors, offering a more accurate starting point than simply guessing or using a market value figure.
Get a Professional Valuation for Complex Properties
For larger, older, listed, or architecturally unusual properties, a professional rebuild cost assessment from a chartered surveyor provides a more reliable figure than a generic online calculator, which the ABI's own guidance acknowledges has limitations for exactly these property types.
Take a Genuine Inventory for Contents
Walking through each room and listing possessions, including estimated replacement values, gives a far more accurate contents figure than an off-the-cuff estimate, and photographing valuable items supports any future claim.
Round Up Rather Than Down
When in doubt, erring on the side of a slightly higher sum insured is generally safer than underestimating, given the proportional impact underinsurance can have through the average clause.
Full Value vs Indemnity: How Payouts Differ
| Scenario | Accurately Insured | Underinsured by 30% |
|---|---|---|
| Total loss (full rebuild needed) | Full rebuild cost paid, up to the sum insured | Payout capped at the (too low) sum insured; shortfall is yours |
| Partial claim, average clause applies | Claim paid in full, subject to excess | Claim reduced by roughly the same percentage as the underinsurance |
| Contents claim, new-for-old cover | Genuine replacement cost paid | Reduced proportionally under average, even for new-for-old policies |
| Ongoing premium | Reflects true risk | Marginally lower premium, but a false economy if a claim occurs |
Reviewing Cover Over Time
Review at Every Renewal
Make reviewing your sum insured a standard part of your annual renewal process, rather than simply accepting an auto-renewal without checking whether the figures still make sense. BCIS's own guidance recommends a full rebuild cost check at least every five years, in addition to routine annual index-linking.
Update After Major Life or Property Changes
Extensions, renovations, major purchases, or significant changes to your possessions are all good triggers to revisit and update your sum insured, rather than waiting for the next scheduled review.
Don't Wait for a Claim to Find Out
The worst time to discover you're underinsured is in the middle of a claim. Proactively reviewing your cover is a small effort compared with the potential financial impact of an inaccurate sum insured.
If You Suspect You're Already Underinsured
Increase Your Sum Insured Mid-Term
Most insurers will let you increase your sum insured at any point during the policy year, not just at renewal, once you've established a more accurate rebuild cost or contents figure. This usually means paying an adjusted premium for the remainder of the term, but it removes the underinsurance risk going forward from that point.
Get the New Figure Properly Evidenced
Whether you're using an updated rebuild cost calculator result or a professional survey, keep a copy of how the figure was reached. This is useful evidence if a future claim, or a future insurer, ever questions the basis for your sum insured.
Understand This Doesn't Fix a Past Claim
Increasing your sum insured today only protects you from this point forward. It doesn't retrospectively change how a claim already settled under the old, underinsured figure was assessed.
Consider Getting Independent Advice for High-Value or Unusual Properties
For larger, older, listed, or non-standard properties where the gap between a rough estimate and the true rebuild cost can be substantial, a broker or chartered surveyor can help you reach a genuinely defensible figure, rather than relying solely on a generic online calculator.
Explore More UK Insurance Guides
Explore insurance information, guides and resources across multiple UK insurance categories.
Browse Insurance GuidesCommon Mistakes and Myths
Myth: Underinsurance Only Matters for a Total Loss
The average clause typically applies to partial claims too. A relatively minor claim can still be reduced proportionally if the underlying sum insured was too low.
Myth: My Sum Insured Was Right When I Set It, So It's Still Right
Rebuild costs move over time, sometimes sharply, as BCIS's own index confirms. A figure that was accurate years ago can now understate true rebuild cost even if nothing about the property has changed.
Mistake: Using Market Value Instead of Rebuild Cost
These are genuinely different figures, driven by different factors. Confusing the two in either direction is one of the single most common causes of underinsurance.
Mistake: Never Revisiting Contents Cover After the Initial Policy
Contents value tends to grow through ordinary purchases over the years. A figure set when a policy first started is rarely still accurate several years later without a review.
Real-World Examples
Example: Kitchen Fire Reveals a Rebuild Cost Shortfall
A homeowner's buildings sum insured was set accurately when the policy began but was never reviewed for several years, a period that included the sharp rebuild cost inflation recorded in BCIS's own index. After a kitchen fire, the insurer applies the average clause because the sum insured no longer reflects true rebuild cost, reducing the claim payout by a meaningful percentage despite the fire itself having no connection to the original valuation.
Example: Extension Never Added to the Sum Insured
A family extends their home with an additional bedroom and bathroom but doesn't update their buildings sum insured to reflect the larger rebuild cost. When a burst pipe causes significant water damage, the insurer identifies the underinsurance and applies a proportional reduction to the settlement.
Example: Contents Underestimated From Memory
A policyholder sets their contents sum insured based on a rough mental estimate rather than a proper room-by-room inventory. After a house fire causes a total contents loss, the true replacement cost significantly exceeds the sum insured, and the payout is capped at the lower figure rather than reflecting what was actually lost.
Frequently Asked Questions About Home Insurance Underinsurance
What is underinsurance in home insurance?
Underinsurance happens when the sum insured on a policy is lower than the true rebuild cost of a property or the true value of its contents, meaning cover may be inadequate at claim time.
What is the average clause and how does it affect a claim?
The average clause allows an insurer to reduce a claim payout proportionally if the sum insured is lower than the actual value at risk, even for a partial claim.
How do I work out the correct buildings sum insured?
Buildings sum insured should reflect the cost to rebuild the property from scratch, including materials, labour, professional fees and site clearance, not its market value, and specialist rebuild cost calculators or surveys can help estimate this.
Does contents insurance also suffer from underinsurance?
Yes, contents underinsurance is common when the total value of possessions is estimated too low, often because people underestimate cumulative value or forget high-value items.
How often should I review my home insurance sum insured?
Reviewing your sum insured at every renewal, and after major purchases, renovations or extensions, helps ensure your cover keeps pace with the true value of your property and belongings. Industry rebuild cost indices show construction costs can move meaningfully within a single year, so even an unchanged property can drift into underinsurance if the sum insured isn't reviewed.
Can I avoid the average clause entirely?
Some insurers offer policies without an average clause, or with more generous terms, but the safest way to avoid its impact is to ensure your sum insured is accurate in the first place.
Why have rebuild costs risen so much in recent years?
UK house rebuilding costs, tracked by the Building Cost Information Service (BCIS) on behalf of the Association of British Insurers, rose sharply during 2021 to 2023 due to materials and labour cost inflation, before moderating to more typical annual increases from 2024 onwards. A sum insured set several years ago, and never reviewed, is very likely to now understate true rebuild cost.
References and Editorial Standards
This guide is reviewed regularly by the ShopTera editorial team to help ensure accuracy and relevance for UK consumers. It is intended for general educational purposes and does not constitute legal, financial or surveying advice. Rebuild costs, insurer average clause terms and index-linking practices vary and change over time; always confirm your specific sum insured and policy terms directly with your insurer or a qualified surveyor.
The BCIS House Rebuilding Cost Index figures referenced in this guide, including the pattern of double-digit annual increases during 2021 to 2023 followed by more moderate increases from 2024 onwards, and BCIS's own recommendation to check rebuild cost at least every five years rather than relying solely on index-linking, are confirmed directly from BCIS's own published index page. Details of the ABI's rebuilding cost calculator and its stated limitations for non-standard properties reflect ABI-published guidance as summarised by insurance industry sources. Broader claims about the prevalence of UK home underinsurance and increased broker-reported claims reductions are drawn from industry survey reporting in UK insurance and broker trade press during 2025 and 2026, and are presented as industry-reported trends rather than official government statistics.
| Date | Update |
|---|---|
| 23 August 2026 | Expanded with BCIS House Rebuilding Cost Index data, a worked average clause example, a full value vs indemnity comparison table, case studies and an expanded FAQ section |
| 31 July 2026 | Initial publication, covering the average clause, rebuild cost vs market value, and contents underinsurance |
Conclusion
Underinsurance is one of the most avoidable, yet most financially damaging, mistakes a homeowner can make. By understanding how the average clause works, setting buildings cover based on genuine rebuild cost rather than market value, taking a proper inventory of contents, and reviewing your sum insured regularly against current rebuild cost data, you can significantly reduce the risk of an unpleasant surprise if you ever need to make a claim.
For related reading, see our Home Insurance UK guide and our What Happens to Your Insurance When You Move House? guide, which explains why a sum insured needs reassessing at a new property rather than simply carrying over.