There are many images and concepts of what the houses of the future could look like, and almost all of them recently appear to be looking at how small a house can be. As you can see from the video below, some people really are taking things to the extreme. The video doesn’t explain certain things such as ‘how do they get water’, and ‘where’s the toilet and how do you flush?’, but those are small things compared to the sort of story you could write about these becoming the norm.
Naturally there would need to be one, two, three and four person variations, or perhaps all families live apart whilst simultaneously living together; it’s up to you, the writer, how you far you want to develop the idea. Have fun.