(1)The provisions of this section shall, subject to subsection (4) of this section, have effect with respect to any planning permission for the alteration or extension of a building on land which is within a controlled area when the planning permission is granted and was also within such an area 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 on or after 1st April 1969.
(2)If the case is the following, that is to say—
(a)either the erection of the building was not development to which these provisions (or Part I of the Act of 1965) applied, or it was such development but no office development permit was required for it; and
(b)either the proposed alteration or extension is not development to which these provisions apply or it is such development but no office development permit is required for it; 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 consequence 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 70(5) or (6) of this Act 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.