Published 2026-07-10 · New Build Mortgage Guide · Written and reviewed by Phillip Wakeling-Smith (CeMAP)

The 28-day exchange deadline: surviving a new-build purchase timeline

Quick answer: Most developers require you to exchange contracts within 28 days of reservation (some say 35 or 42 — the pressure is the same). Miss it and you can lose the plot and your reservation fee. The deadline is beatable, but only if the mortgage work starts before you reserve, not after.

Why builders do this

A reserved-but-not-exchanged plot is dead stock: the developer can't sell it, but you're not yet committed. Their sales targets run monthly, so they compress your side of the transaction into one. It also, frankly, keeps buyers from shopping around once emotionally committed. Whatever the motive, the contract terms are rarely negotiable — your protection is preparation.

Where the 28 days actually go

A mortgage application involves valuation booking, underwriting, and (on new builds) extra paperwork — warranty documents, the Disclosure of Incentives Form, management company details on flats. Conveyancing on a new build is heavier than on a second-hand home, not lighter: your solicitor is reviewing a developer's contract pack, planning conditions, road adoption agreements and estate covenants. Twenty-eight days is tight for all of that even when nothing goes wrong. A single surprise — a down-valuation, a slow employer reference, a solicitor holiday — and it isn't enough.

How to make the deadline comfortable

Get a real decision in principle first — a credit-checked DIP from a lender whose new-build criteria fit your plot type, not a soft-quote from a comparison site. Do this before you visit the sales office.
Use a new-build-experienced solicitor — developers publish panels of firms who know their contract packs. You're not obliged to use them, but whoever you pick must do new-build volume; a high-street generalist learning on your file will not hit 28 days.
Have documents ready on day one: payslips, bank statements, ID, proof of deposit and — if any of your deposit is gifted — the gift letter. Days 1–5 are where most buyers quietly lose the race.
Declare the incentives immediately so the valuation isn't re-run when the Disclosure of Incentives Form surfaces late.
Ask for the extension early, not at day 26. Developers grant extensions more readily when the delay demonstrably isn't yours and you flag it in week two.

The long-stop question nobody asks

Exchange is your deadline; completion is the builder's — and on off-plan purchases it may be months or years away, governed by a "long-stop date" in the contract. Two things matter: that a long-stop date exists, and that your mortgage offer's validity covers the realistic build timeline (new-build offers commonly run 6–9 months from the outset, with further extensions available from many lenders). If the build overruns your offer, you're reapplying under whatever criteria and rates exist then — plan for that possibility from the start.

This article is information, not financial advice. Contract terms vary by developer — your solicitor's review is the only one that counts.