Rebuild cost indexation: what it updates and what still needs checking
- RebuildCostASSESSMENT.com
- Aug 17
- 4 min read
A bigger cash gap does not always mean a bigger percentage gap. That simple distinction helps explain what indexation updates and what still needs checking at renewal.
One figure, two jobs
At renewal, indexation updates an existing rebuild figure using the method set out in the policy. It does not check how that figure was calculated or whether the property has changed since it was set.
Put simply, indexation updates the number. It does not reassess the property. If the starting figure and the index are both suitable, it can be useful between reviews.
How annual adjustments build up
For this example, assume each annual adjustment is applied to the previous year’s result. That makes the movement multiplicative: each increase becomes part of the starting point for the next one.
Take a purely illustrative starting figure of £500,000. A 4% adjustment in the first year would take it to £520,000. The next 4% would apply to £520,000, producing £540,800. After five annual adjustments, the figure would reach approximately £608,326.
Simply adding five adjustments of 4% gives 20%. Because the increases build on one another, the cumulative movement is about 21.7%.
The 4% rate is hypothetical. It is not a current rebuilding-cost rate, a forecast, a recommendation or RebuildCostASSESSMENT.com data. Policy mechanisms vary, so the wording and insurer information for the policy should determine how indexation works in practice.
A bigger cash gap can sit alongside the same percentage gap
Now let Figure B start 10% lower, at £450,000. If both figures receive the same 4% adjustment each year, this is what happens:
Illustrative example using hypothetical assumptions
Illustrative figure | Starting point | After five annual 4% adjustments |
Figure A | £500,000 | £608,326 |
Figure B, 10% lower | £450,000 | £547,494 |
Approximate cash difference | £50,000 | £60,833 |
Percentage difference | 10% | 10% |
After five years, the cash gap is about £10,833 larger. Yet Figure B is still 10% below Figure A because both have moved by the same factor.
The gap has grown in pounds, but not as a percentage. Saying that the original 10% difference has “compounded” would blur that distinction.
In this example, the percentage gap changes only if the two figures are adjusted differently. In practice, an indexed policy figure can move away from a new property-specific estimate for other reasons. The index may not suit the building, the property may have changed, or the information behind the original figure may be incomplete or out of date.
The example only illustrates the maths. It does not assess a real property, say whether any sum insured is adequate or predict how a policy would respond.
What indexation can do
Each index is designed to track a particular type of cost change. The Building Cost Information Service (BCIS) describes its House Rebuilding Cost Index as designed to update residential rebuild costs between assessments. It says its All-in Tender Price Index is commonly used to update non-residential rebuild costs between professional assessments. Neither replaces periodic reassessment.
Policies do not all apply indexation in the same way. The Royal Institution of Chartered Surveyors notes that the figure being indexed and the timing of the adjustment can vary. Brokers should therefore check the policy wording and insurer information.
Consumer-price inflation (CPI) is different again. The Office for National Statistics defines CPI and CPIH around goods and services bought for household consumption. That is not the same scope as reinstating a building, so a headline consumer-inflation rate is not a stand-in for rebuild-cost movement.
What indexation cannot check
Indexation can only work with the number it starts with. It cannot tell whether that number came from a professional assessment, an estimate, market value, a historic record or another source. If there is a problem with the starting figure, indexation carries that problem forward.
Nor can it spot an extension, a change of use or outdated property information. A percentage adjustment cannot reflect information it has not been given. BCIS treats current property information as a separate part of maintaining reinstatement figures over time.
This is why indexation, review and professional reassessment complement one another. Indexation moves the figure. A review checks the supporting information. If that evidence no longer supports the figure, further assessment may be worth considering.
Three checks at renewal
1. Which starting figure is being carried forward?
Trace the number back to its source. A professional rebuild cost assessment, an estimate, market value, a historic record and an unknown source do not provide the same basis. Indexation does not resolve uncertainty about where the figure came from.
2. What exactly has been indexed, and when?
Is the declared value or sum insured being adjusted? Does the movement apply at renewal, during the policy period or at another point? Which index or method is being used? RICS explains that these mechanisms vary, so “index-linked” is not the whole story. Check the policy wording and insurer information.
3. What sits outside the index?
Look for material alterations, a change of use, missing or outdated information, and any mismatch between the index and the exposure. An index can only reflect the movement it was designed to measure.
These checks are conversation prompts, not a fixed decision tree. The appropriate next step depends on the property, the available information, the policy wording and the circumstances.
Keep the two jobs separate
At renewal, ask three simple questions. Where did the figure come from? How has the policy moved it? Has anything changed that the index could not see? The answers help show whether routine indexation is doing its intended job or whether the figure deserves a closer review.
Choose the right next step
Need a new assessment? Arrange a Rebuild Cost Assessment for your client. Desktop Refresh may be available if the property has not changed and the existing RebuildCostASSESSMENT.com desktop report is less than three years old. It is only available to active broker account holders.
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This article provides general information only. It is not financial, insurance, legal or property advice. Decisions about cover should take account of the relevant policy wording and individual circumstances.
