That's something that often gets pulled in together, however, Building Regulation compliance and getting a build signed off are separate to planning approval.
Planning approval is pretty much a, 'Yes, it's OK to build X houses/flats/whatever in this location, the appearance looks fine, it has been agreed by all the statutory consultees, etc, etc. It fits in with the Local Plan, Secretary of State is happy.'
Building Regs sign off is very much, 'Is the cladding right, are the fire doors right, does it meet insulation and vent standards, is the wiring tested, is the plumbing/drains installed correctly, etc, etc. Is it the agreed sizes, is it in the right place, etc.'
So. yes, regulations around fire and safety, etc, can change during a build but the core planning permission is valid from the day it is agreed. It is very, very, very difficult to the extent of being almost impossible to revoke a granted planning permission. You only have to look at the Council's attempt to pull the permission for the Marine Plaza development opposite the Kursaal to see how difficult and convoluted it is, and the Council pretty much got their rear end served to them on a plate in the Courts...