If you try to sell your house and garden separately to keep both under particular stamp duty bands they may be classed as linked by HMRC:-
"SDLT for linked purchases or transfers
There are some situations when two or more property transactions that involve the same buyer and seller are treated as being 'linked' for Stamp Duty Land Tax (SDLT) purposes. People connected to a buyer or seller can count as being the same buyer or seller. If two or more transactions are treated as linked then the buyer must pay any SDLT due on the total value of all the linked transactions. This may mean that they have to pay a higher rate of SDLT than if the transactions had been treated individually.
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For example, if a residential property was sold in such a way that Mrs Smith purchased the house but her husband Mr Smith bought the garden, the two transactions would be linked. This is because Mr and Mrs Smith are connected people and they're buying things from the same seller as part of a single deal..
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For example, a property speculator might agree to buy three new houses from a builder. The builder offers a special price for the houses because the speculator agrees in advance to buy three. It's agreed that the buyer will pay £180,000 for each house once it's complete.
In this example, the three transactions are linked as part of a series. So SDLT is payable on the total chargeable consideration of the three transactions - £540,000. The buyer pays SDLT at 4 per cent on £540,000, which is £21,600. If each individual transaction has a different effective date, a separate SDLT return will be needed for each. "