Hello,
Option 2 would be my choice.
I would install a switch fuse at the intake position in the main building to supply yoy treatment rooms rated at 63a. If you should wish to sub meter at a later stage it's easily done. The retain the existing consumer unit in the house. The main isolator will of course, control both buildings.
The earthing arrangement will be determined by your electrical engineer.
Regards,
NA
Hi,
This does seem to be the path of least resistance in terms of interference from the main house. The items being used in the treatment room will mainly be the shower and heating in the winter. The lighting is all LED, and the balance of mains usage will be negligible in terms of laptop etc.