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HMRC internal manual

Stamp Duty Land Tax Manual

Scope: When is Stamp Duty Land Tax (SDLT) chargeable: Contracts and substantial performance: Substantial performance at completion FA03/S44(3)

If a contract is substantially performed only at the time it is formally completed, then the contract and conveyance comprise a single land transaction. In this case, the effective date for the purpose of SDLT would be the date of completion.