The sort of advantages you should look at when buying a property with a granny flat, or designing one yourself should, IMO, include:
potential wheelchair access not only to the unit itself, but throughout the unit. Hobless shower with sliding or folding door (if a door at all), bars in shower and by the toilet for ease of getting up/remaining standing.
Lightswitches and power sockets at a convenient distance from the ground (lightswitches lower than normal, sockets higher).
The kitchen needs special attention. Perhaps design it so that it will be easy to lower some of the benchtops so that if a wheelchair is required later, the occupant can slot under the sink for washing up or preparing vegetables. Cooking rings should be all at the front of the benchtop.
