Home Elevator

[Gwaithmir]

You may be right but you should probably get a professional assessment before disregarding that option. Stair lifts are designed with private residences in mind while elevators are more a thing for commercial buildings.

If the stairs are built to code it should be no problem. If both sides are accessible reinforcing the whole structure would be easier.

Have you considered replacing the stairs with an vertical lift? The business end of a forklift would  work. (Don't ask me how I know that, and don't ask where that missing forklift went, m'kay?   )

Hoyt home elevators are chain driven and lift all four corners of the cab for greater stability. This design is a bit more expensive but, from a mechanical engineering viewpoint, I have more confidence in it.