(1) Building work to alter an existing building must comply with these Regulations.
(2) Subject to regulations 234 and 236, if the proposed alterations to an existing building, together with any other alterations completed or permitted within the previous 3 years, relate to more than half the original volume of the building, the entire building must be brought into conformity with these Regulations.
(3) Despite subregulations (1) and (2) and subject to subregulation (6), the relevant building surveyor may consent to partial compliance of building work or an existing building with subregulation (1) or (2).
(4) In determining whether to consent to partial compliance with subregulation (1) or (2) in respect of any alteration to a building, the relevant building surveyor must take into account—
(a) the structural adequacy of the building; and
(b) the requirements necessary to make reasonable provision for—
(i) the amenity of the building and the safety and health of people using the building; and
(ii) avoiding the spread of fire to or from any adjoining building.
(5) Any consent to partial compliance under subregulation (3) must be in the form of Form 18.
(6) If any part of the alteration is an extension to an existing building, the relevant building surveyor may only consent to partial compliance in respect of the extension if the floor area of the extension is not greater than the lesser of—
(a) 25% of the floor area of the existing building; and
(b) 1 000 m 2 .