CG64300 | Private residence relief: the entity of the dwelling house: dwelling-house is the whole building
From HM Revenue & Customs · Capital Gains Manual
Private residence relief is only available in respect of a gain accruing on the disposal of, or of an interest in, a dwelling-house which has been occupied as the only or main residence.
In most cases the whole of a building in which a person has a residence will be a dwelling-house. This will include:
Premises in which the owner resides although he or she also carries on a business there (for example rooms above a shop)
Premises which are partly let as residential accommodation
Small hotels and guest houses where the proprietor lives on the premises.
In such cases, although the whole of the building may be the dwelling-house, it is only partially used as a residence. Therefore, private residence relief will be restricted to only relieve the part of the gain accruing which is attributable to the residential part of the dwelling-house (see CG64650+).