A client whose property sum insured has not been reviewed in two to three years could already be carrying a shortfall of 10% or more against current reinstatement values - before any claim is filed. That gap has been widening quietly through the same period that rebuild costs have been rising. This summer's weather pattern has made it urgent.
The BCIS General Building Cost Index recorded annual growth of 3.9% through July 2026. Compounded over two or three years of deferred review, that rate produces a material coverage gap even on properties where nothing has physically changed. When storm or flood damage then requires a claim, the shortfall between what a policy will pay and what reinstatement actually costs becomes visible for the first time - at the worst possible moment.
LexisNexis Risk Solutions data puts a number on the current flood risk picture: seven in 10 UK home insurance flood claims are now linked to sudden, intense rainfall rather than rivers overflowing their banks. Nine in 10 of those surface water claims come in under £5,000. River flooding is rarer but typically twice as costly when it occurs. The average flood, storm and burst-pipe claim reached £8,548 in Q2 2026.
The physical mechanism behind the surface water surge is well-established. Prolonged dry, hot weather bakes the ground to a state where it cannot absorb sudden heavy rainfall. When the pressure breaks into thunderstorms - as it has done repeatedly this summer - the water runs off rather than soaks in, producing surface water flooding in areas that would not register as flood-risk on traditional river-proximity maps.
Britain recorded its fifth heatwave of 2026 in mid-August, with temperatures reaching a provisional 38.1°C at Kew Gardens on August 13 - the fifth hottest day ever recorded in the UK, with 40 locations across central and southern England exceeding 36°C. A yellow thunderstorm warning followed, consistent with the summer's established cycle of heat building into intense rainfall. The Met Office has signalled that changeable, thundery conditions are likely to continue into autumn.
Cos Kamasho, principal consultant at BCIS, connected the weather pattern directly to the renewal conversation brokers should be having now.
"Five heatwaves followed by heavy rainfall and the risk of flash flooding bring both the physical threats of extreme weather and property underinsurance into much sharper focus," Kamasho said.
"The more extreme and changeable the weather, the greater the potential for damage-related claims to expose existing underinsurance. That is why using robust, up-to-date rebuild cost data to inform sums insured and support regular reviews is absolutely critical."
He added that brokers and insurers are in the best position to help clients make proactive choices at policy inception and renewal, with access to rebuild cost data that clients typically do not have themselves.
For brokers with residential or commercial property clients, the renewal prompt is specific. A sum insured set in 2022 or 2023 - before the most recent run of BCIS cost index growth - is likely to be understating current reinstatement value. The question to ask at every property renewal this autumn is not whether the client has made any physical changes to the building, but whether the sum insured has kept pace with what it would actually cost to rebuild it now.
Surface water flood risk adds a second dimension to that conversation. A client in an area that does not appear on traditional flood-risk maps may still be exposed to the kind of intense rainfall event that has driven seven in 10 UK flood claims this year. Confirming that surface water flood is covered - rather than assumed to be covered - and that the sum insured is adequate against current rebuild costs, are the two questions that should be on every property renewal checklist before the next downpour arrives.
