(1)The provisions of this section shall, subject to subsection (4) below, have effect with respect to a planning permission granted after the commencement of this section for the alteration or extension of a building in an area to which Part I of the Act of 1965 applies at the time of the grant and also applied when the application for planning permission was made, but shall have effect only in the case of a building erected under a planning permission granted after the said commencement.
(2)If the case is the following, that is to say:—
(a)either the erection of the building was not development to which Part I of the Act of 1965 applied or it was so but no office development permit was required therefor; and
(b)either the proposed alteration or extension is not development to which the said Part I applies or it is so but no office development permit is required therefor ; and
(c)there will result from the proposed alteration or extension a building with an aggregate floor space of twice, or more than twice, the prescribed exemption limit,
the planning permission for the alteration or extension shall be granted subject to the condition specified in subsection (3) of this section (in addition to any other conditions imposed by the authority granting the permission).
(3)The said condition is that the use of the building as altered or extended, or as subsequently further altered or extended, shall be restricted so that (whether in consquence of a change of use or otherwise) it does not at any time contain office premises having an aggregate office floor space which exceeds the prescribed exemption limit.
(4)In the following two cases this section shall not apply:—
(a)where the planning permission is in respect of a building which, after its alteration or extension, will be wholly residential; and
(b)where the planning permission is subject to conditions by virtue of section 23(5) or (6) of the [1966 c. 34.] Industrial Development Act 1966 and those conditions either restrict the office floor space which the building as extended or altered may contain or preclude it from containing any office floor space.