IIRC it's required for larger houses but I can't remember the size
threshold
or what reg it's in (part L, I'd guess)

As I understand it, separately timed zones are suggested for floor areas
exceeding 150m2. However, temperature control for separate living and
sleeping areas is always required (usually using TRVs or subzoning) unless
the flat is open plan and such control would be pointless.

Christian.