Here is a cut-out-and-keep case which provides evidence not only that permitted development rights do not apply to a development which has yet to be completed, but also that they cannot apply in anticipation of completion of the development.
The case concerns a basement extension in central London (DCS Number 400-010-068). The appellants sought a certificate of lawfulness stating that a rooflight would constitute permitted development, subsequent to the completion of the development. The planning inspector explained, however, that his assessment must be based on whether or not the operations proposed to be carried out would be lawful if begun at the time of the application. In this respect he referred to the decision of the High Court in R. (on the application of Townsley) v. Secretary of State for Communities and Local Government  in which Mr Justice Collins stated “ “It seems to me quite plain that the GPDO cannot be used to effect a change in what is permitted by a planning permission when that planning permission is being carried out. The purpose of the GPDO is to allow alterations and extensions or amendments to an existing building which has already been constructed.” As the basement extension had not been constructed the inspector concluded that the proposed operations could not have been lawfully begun at the time of the application.
The following DCP chapter is relevant: 4.343