Private residence relief: the entity of the dwelling house: dwelling house in part of a building: self contained units
There are also buildings which although appear from the outside to be a single dwelling house, they are in fact split up into separate self contained units. Each self contained unit is itself a dwelling house.
Whether or not such a self contained unit exists is a question of fact. In order to be a separate dwelling house the unit will have all the facilities one would expect in a home, such as kitchen, bathroom, bedroom, etc. The unit might be structurally separate from the rest of the building like a flat or it may be merely sealed off. It will usually have a separate access to the outside or to an outer door or common staircase. It should also be capable of separate sale.