2The said value shall be calculated by reference to prices current at the time in question—
(a)as if the development had not been initiated but the land had remained in the state in which it was immediately before the development was initiated ; and
(b)on the assumption that (apart from the principal Act) the development could at that time lawfully be carried out,
and shall be taken to be the difference between the value which in those circumstances the land would have had at that time if permission for that development had been granted unconditionally immediately before that time and the value which in those circumstances the land would have had at that time if permission for that development had been applied for and refused immediately before that time, and it could be assumed that permission for that development, and any other new development of that land, would be refused on any subsequent application:
Provided that, if the development involved the clearing of any land, the reference in sub-paragraph (a) of this paragraph to the state of the land immediately before the development shall be construed as a reference to the state of the land immediately after the clearing thereof but before the carrying out of any other operations.