When I did the same thing in the last house (subsequently sold with no comment) it was told to me that a detached outbuilding that was exempt from building regs due to its size and location, remained exempt if you modified the internals, as long as you didn't add sleeping accommodation. AFAIK garden rooms, offices, summerhouses etc <30sqm are generally exempt from BR (with some qualifications regarding height, proximity to boundary etc). If you add electrics, plumbing -these do come under BC, but only for those elements of work.
As I say, I subdivide my detached garage on the last house to make a little work area, and during the sale (Sep 19) with survey etc, no one batted an eyelid at it.