There is no single legal definition of "habitable room", as its use and meaning is subject to context.
For example, the Building Regulations Approved documents provide 3 separate definitions in different parts:
- Part B: A room used, or intended to be used, for dwellinghouse purposes (including for the purposes of Part B, a kitchen but not a bathroom).
- Part F: A room used for dwelling purposes but which is not solely a kitchen, utility room, bathroom, cellar or sanitary accommodation.
- Part M: a room used, or intended to be used, for dwelling purposes including a kitchen but not a bathroom or utility room.
There are also separate definitions in regards to calculation of residential density, and although they share common themes, the definitions vary and you are advised to check with your Local Planning Authority as to how the calculation should be made.