For their dream home, my mother and stepdad had it designed to include a small elevator between the two levels. Probably the most forward-looking design decision they made on that house, as my mother uses it every day. Doesn't need to, but with aching knees it really makes life easier.
It's always easier to plan ahead and build something in from the start than it is to remodel. It's an absolute nightmare to bring a century-old building up to modern code for structure and accessibility.