This part of GOV.UK is being rebuilt – find out what beta means

HMRC internal manual

Stamp Duty Land Tax Manual

Example: P is buying a house for £2 million

P is buying a house for £2 million. One room is used as an office, but there have been no alterations made which would make it unsuitable for use as part of a residence.

Because that room is suitable for use as a dwelling (S116(1)(a)/FA03) the whole property is residential. Table A applies so the rate of tax is 5 per cent.