Category 1 (conventional house, containing one dwelling) covers detached/semi-detached/terraced houses and duplex blocks (a three-storey building with apartments on the ground floor and two-storey townhouses/duplex units on the two upper floors ... each dwelling unit has its own external entrance) provided they each contain only one dwelling unit regardless of whether vacant or not. In general, such units will be in their original purpose built state and will have an unshared entrance and a unique address.

Category 2 (one dwelling building, partly non-residential) typically covers a caretaker's flat in an office block, a lone flat above a shop, a lone flat in a conventional house where the remainder is being used as office or other business accommodation. It is important to note that this Category does not apply if more than one private dwelling unit, whether vacant or not, is in the building ... Category 3 applies in such circumstances.

Category 3 (multi-dwelling building) includes apartment blocks (and purpose built blocks of flats, private or Local Authority) and former conventional houses which now contain two or more dwelling units.