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HMRC internal manual

Stamp Duty Land Tax Manual

Scope: What is chargeable: Land transactions: Main subject matter FA03/S43(6)

Main subject matter means the chargeable interest acquired in a land transaction and includes any interest or right relating to it.

So, for example, the transfer of a freehold and the transfer or creation or variation of a related right of way is chargeable as one transaction rather than two.