Ramp won’t work due to slope of the yard and other factors. When we built the house 12 years ago we designed the first floor for handicapped access and use ... but didn’t think far enough ahead to put the elevator in then. But we also delayed the cost til now.
Could you use a lift rather than an elevator?
There’s another alternative: You could move downstairs and give them the upstairs. It might be difficult, but it’s temporary, after all.