House adaptions

His own area is an excellent idea.

First thought would be his independent access to upstairs for the future. Ie room ultimately for thru floor lift.

I too had plans for similar but was abandoned as council couldn’t figure out lift access for future needs.
I DIY built a thru lift out of boxes to dimensions and placed where the council thought it should go. would have lost use of living room and room above as it needed to go pretty much middle of room

I would advice downloading a room planner app for pc and fitting out your rooms with what you need as council dont always consider actually living in the areas.

the benefit of fitting out downstairs or even an extension means no lift breakdown hassle, although the first floor self contained flat option would create more privacy for him and yourself if he has carers in .