“Where a proposal relates to a building or use which the applicant is expected to retain or continue only for a limited period, whether because they have specifically volunteered that intention or because it is expected that the planning circumstances will change in a particular way at the end of that period, then a temporary permission may be granted. For example, permission might reasonably be granted on an application for the grant of a temporary building to last seven years on land which will be required for road improvements eight or more years hence, although an application to erect a permanent building on the land would normally be refused”