That depends on how much is needed. I've lived in houses with doors I need to turn sideways to get through (and I'm a wide guy - I expect fat people couldn't fit in any direction. I've lived in houses where all doors are bit enough for a wheelchair. Most houses have a range of sizes - what is your requirements. Some things are "easy" to do, and some are a major tear down. Without knowing the exact size requirements of a real robot we can't have this discussion. Though If you are in the business I expect you to look at houses and try to come up with something reasonable unless there is a good reason you can't.

There are always first adopters who will if it isn't too expensive. Rich people often live in a house with big enough doors (well most doors big enough - enough for this discussion to say they do), and likely have a need (and the money) for something even if it isn't very good. If your robot starts to prove useful people will ask for it - we build thousands of new houses/apartments every year just in the US, if builders see a demand they will make changes to their new models - it doesn't cost that much more so they don't even need a real demand, just a marketing feature that you could even if nobody does will work (for a few years: you better prove it useful to those who buy new houses fast or the fad will pass!)

It isn't easy because as you say, chicken and egg. If these prove useful people will make changes of the next few decades so they can have one. (though of course your competition will be looking to see if they can make something that doesn't need a remodel to use)