Quick summary:

Day One Uplift and Index Linking are the two main ways a buildings insurance policy protects a block of flats against rising rebuild costs.

Day One Uplift adds an agreed percentage buffer to the sum insured at the outset of the policy and that buffer keeps working right through a claim; including a rebuild that drags on for two or three years.

Index Linking adjusts the sum insured in line with a building cost index during the policy year, but that adjustment stops at the point of loss. For small claims the difference is often invisible.

For a total loss on a block of flats, it can mean the difference between a full settlement and a shortfall running into hundreds of thousands of pounds.

Why This Matters for Blocks of Flats Specifically

Blocks of flats are expensive and slow to rebuild. A serious fire, storm, or escape of water affecting a whole building can mean:

• A rebuild programme of two to three years, sometimes longer once planning, party wall agreements and specialist trades are factored in.
• Construction cost inflation that can move sharply within that window; material costs, contractor day rates and scaffolding/access costs have all shown double-digit swings in recent years.
• A sum insured that was accurate on day one of the policy but is badly out of date by the time reinstatement actually finishes.

This is exactly the scenario where the mechanics of Day One Uplift versus Index Linking stop being small print and start being the difference between a manageable claim and a funding gap that leasehold owners have to cover themselves.

The Core Difference in One Sentence

• Day One Uplift: an agreed percentage (commonly 15%–50%) is added to your declared reinstatement cost from the start of the policy, so the extra protection is available from day one of the policy and stays available throughout the life of the claim, however long the rebuild takes.

• Index Linking: the sum insured is periodically adjusted (often quarterly) in line with a recognised building cost index, but that adjustment only tracks costs up to the point of loss. Once the claim happens, the index-linked figure is fixed; i.e. it doesn’t keep moving with construction inflation while the building is actually being rebuilt.

That’s the practical distinction the rest of this article works through.

How Each One Works

Day One Uplift:

You (or your managing agent/broker) declare an accurate rebuild cost for the block at the start of the policy. The insurer then applies an uplift percentage on top of that figure, say 30%, as a built-in cushion. Provided the underlying declared value was accurate, most policies on a Day One basis also waive the “average” (underinsurance) clause entirely, up to the limit of the uplift.

This means:

• The buffer exists from the first day of cover, regardless of when in the policy year a claim happens.
• It continues to apply through the claims and rebuild process — so if reinstatement takes three years and costs rise well beyond what anyone predicted at renewal, the uplift is still there to absorb it (up to its percentage limit).

Index Linking:

The sum insured is automatically increased in small increments through the policy year, tracking a construction/building cost index (such as a RICS or BCIS-style index). This keeps the figure reasonably current while the policy is live and no claim has happened.

The catch: once a loss occurs, the index-linked sum insured is crystallised at whatever the index says on that date. It does not carry on rising with real-world rebuild costs for the months or years it actually takes to complete the works. If inflation during the rebuild period outpaces what the index captured beforehand, that gap is not covered.