This sounds peculiar. As mentioned, surely, they’d first need Local Authority permission. You’d think the location of plumbing and services, as well as things like bedroom window provision would come into it, as well as minimum allowable floor space per the number of bedrooms. Things like this matter, otherwise you get folks trying to convert their lock-up garage into a bedsit or convert a one bedder into a 5 bedder if they can get away with it.