In this guide
6. Convert a space above a shop into a flat
The regulations define this as a 'material change of use' and specify the requirements with which, as a result of that change of use, the whole or part of the building must comply (e.g. those concerned with escape and other fire precautions, hygiene, sound insulation, and energy conservation).
The whole, or at least part, of the building may therefore need to be up-graded to make it comply with the specified requirements.