Garage to coach house

[pumpingRSI]

We have a v large garage, double one side, then large is outbuilding workshop attached. It has a floor above into eaves but not quite head height.

It's not attached to house and several metres away.

It's suffering from big cracks and general issue with build so would live to rebuild in future into garage with accommodation to one side and poss above garage. Does anyone know with planning whether we'd be allowed to rebuild to slightly taller height to accommodate a floor above?

Anything is possible but your best port of call for this is your local planning office.
They hate increasing height but then can make some truly odd decisions too so definitely book an appointment.
Have you considered a lower ceiling height and a mezzanine as a potential solution?