Am I under-insured? What Underinsurance Means for a Flood or Fire Claim

Underinsurance is the most widespread and least understood problem in UK home insurance. According to data published in January 2026, 70% of UK properties remain insured below their true rebuild cost. The average shortfall is 34%, meaning the typical under-insured property is covered for just 66% of what it would actually cost to rebuild.

For most policyholders, this gap only becomes visible at the worst possible moment: when they make a significant claim following a flood, fire, or major water damage event, and discover their settlement is proportionally reduced by the same percentage they were under-insured.

This guide explains how underinsurance works, why it happens, how to find out if your property is affected, and why it matters specifically in the context of flood and fire restoration claims.

The Difference Between Market Value and Rebuild Cost

This is the core confusion that causes underinsurance for most UK homeowners.

Market value is what your property would sell for on the open market. It includes the value of the land, the location, the neighbourhood, supply and demand conditions, and all the factors that determine what a buyer would pay.

Rebuild cost, also called reinstatement cost, is what it would cost to demolish and completely reconstruct the building from scratch to its current specification. It includes materials, labour, professional fees (architects, structural engineers, planning consultants), debris removal, and compliance with current building regulations.

These two figures are not the same. They are often significantly different. A property worth £600,000 on the market in London might have a rebuild cost of £350,000, because the land value accounts for a large proportion of the market value. Equally, a period property in the countryside with extensive stone construction might have a market value of £450,000 and a rebuild cost of £650,000, because the specialist labour and materials required to reconstruct it are disproportionately expensive.

Your buildings insurance should be set at the rebuild cost, not the market value. Insurers require this. If you have used the market value as a proxy for your sum insured, which many people do, you may be significantly under-insured or overinsured depending on which way the values diverge in your specific case.

How the Average Clause Works: The Maths That Matters

When a property is under-insured and a claim is made, most UK home insurance policies contain a provision called the average clause (also referred to as the Condition of Average). This clause allows the insurer to reduce the claim settlement proportionally to reflect the degree of underinsurance.

The formula is:

Claim payout = (Sum insured ÷ True rebuild cost) × Claimed loss

A worked example makes this concrete.

A property has a true rebuild cost of £500,000. The policyholder has insured it for £400,000, 80% of the correct figure. A flood causes £60,000 of damage to the ground floor. The average clause is applied:

(£400,000 ÷ £500,000) × £60,000 = £48,000

The policyholder receives £48,000, not £60,000. The shortfall of £12,000 comes directly from their pocket, despite them having paid their premium and believing they were covered.

Critically, the average clause applies to every claim, not just total losses. Even a relatively modest claim, replacing damaged flooring, drying out walls, reinstating plaster, is subject to the proportional reduction if the policy is under-insured.

Why Underinsurance Is So Common

Given how significant the consequences are, why do so many properties end up under-insured?

The market value confusion is the most common cause. Many policyholders simply use their property’s market value as the sum insured, not realising that rebuild cost is a separate and unrelated calculation.

Outdated valuations. Rebuild costs in the UK were estimated to be 35 to 40% higher in early 2025 than they were in 2020, driven by construction cost inflation, supply chain disruption, and rising labour costs. A valuation carried out five years ago, even if it was accurate at the time, may no longer reflect current rebuild costs. Index-linking alone has not kept pace with this level of inflation.

Forgotten improvements. Extensions, loft conversions, kitchen refurbishments, and structural alterations all increase the rebuild cost of a property but are frequently not reflected in an updated sum insured. Adding a rear extension worth £80,000 to construct without updating your buildings insurance is a common cause of material underinsurance.

Online calculators. Free rebuild cost calculators provide indicative estimates but are not designed for setting insured values. They cannot account for unusual construction, period features, specialist materials, basements, or local variations in labour costs. Relying on one for your sum insured is a material risk.

The Specific Problem in Flood and Fire Claims

Flood and fire claims are precisely where underinsurance causes the most damage, because these are the events that produce the largest reinstatement costs.

A significant flood can require:

• Full structural drying of all affected rooms, often running for several weeks
• Replacement of all floor coverings throughout the affected area
• Replastering of walls following the drying programme
• Replacement of kitchen units, fitted furniture, and built-in appliances
• Redecoration throughout
• Specialist treatment of any timber elements affected by prolonged moisture

A major fire can require full rebuilding of affected sections of the structure, smoke and soot remediation throughout the property, replacement of all affected contents, and months of alternative accommodation.

These are exactly the categories where an average clause reduction of 20 or 30% produces a five-figure shortfall that the policyholder has to fund themselves, while simultaneously displaced from their home and navigating the stress of the restoration programme.

For properties in London, where rebuild costs per square metre are among the highest in the country, the financial exposure from underinsurance is proportionally greater.

How to Find Out If Your Property Is under-insured

There are three approaches, in order of reliability.

Commission a RICS rebuild cost assessment. This is the most reliable method. A RICS-qualified surveyor assesses the property, calculates the accurate reinstatement cost, and produces a report that provides the correct figure to use as your sum insured. Most standard properties can be assessed for between £150 and £300 as a desktop assessment. Complex, period, or high-value properties may require an on-site inspection.

Use the BCIS House Rebuilding Cost Calculator. The Building Cost Information Service calculator, available on the ABI website, is the most widely accepted free calculator for residential properties. It is more robust than generic online tools and takes into account property type, floor area, construction type, and regional factors. It is not a substitute for a RICS assessment for unusual or complex properties, but provides a reasonable starting point for standard residential stock.

Review your policy sum insured against recent sold prices and construction costs in your area. This is the least precise method but a useful sanity check. If your buildings sum insured is close to or above your property’s market value, this is a strong signal that something is likely wrong, either the sum insured is inflated (if rebuild cost is lower than market value in your area) or inadequate (if rebuild cost is higher).

What to Do If You Discover You Are under-insured

If a rebuild cost assessment reveals a meaningful gap between your current sum insured and the correct figure, the steps are straightforward.

Contact your insurer and request an amendment to your sum insured. Your premium will likely increase, by how much depends on the extent of the uplift and your policy terms. This is a cost worth bearing: the premium increase for correcting a 20% underinsurance gap is substantially less than the out-of-pocket shortfall on a significant claim.

Update your sum insured every three to five years and immediately following any significant building work, extensions, conversions, structural alterations, major kitchen or bathroom refurbishments.

If you are mid-claim and believe underinsurance is being applied unfairly, the next section is relevant to your situation.

For the ABI House Rebuilding Cost Calculator, check: ABI, rebuilding cost calculator

How a Professional Scope of Works Reduces Underinsurance Exposure

Underinsurance affects the quantum of the payout, the total amount the insurer will pay. What a professional restoration company’s documentation affects is the accuracy and completeness of the claim within that quantum.

An itemised scope of works produced by Disaster Care Capital documents every element of the reinstatement, structural drying, material replacement, plastering, decoration, specialist treatments, priced at current market rates and in a format loss adjusters accept without dispute. This ensures the claim captures the full extent of the damage accurately, minimising the risk that items are missed, undervalued, or excluded from the settlement scope.

Where underinsurance has already been established, this documentation becomes even more important, because the proportional reduction applies to the claimed figure, not an inflated one. Every pound of legitimate damage that is correctly documented and included in the claim is a pound to which the average clause applies, rather than a pound lost through incomplete claim compilation.

Disaster Care Capital produces fully costed scopes of works, moisture mapping reports, and daily drying logs that insurers accept at first presentation. Call us today for a free same-day assessment.

For guidance on insurance complaints and underinsurance disputes, check: Financial Ombudsman Service, insurance

Conclusion

Underinsurance is not a fringe issue, 70% of UK properties are currently insured below their true rebuild cost, with the average shortfall meaning homeowners are covered for only 66% of what a full rebuild would require. In a flood or fire claim, this translates directly into a five-figure or greater shortfall that falls on the policyholder.

The solution is to understand the difference between market value and rebuild cost, commission a RICS rebuild cost assessment, and update the sum insured regularly. The premium cost of adequate cover is a fraction of the financial exposure of the average clause in a significant claim.

Disaster Care Capital documents the full scope of reinstatement works accurately and completely, ensuring that whatever the quantum of your claim, every legitimate element is captured, evidenced, and presented to your insurer without dispute.

Frequently Asked Questions