I think that the cost issues are due to the fact that most people nowadays don't have households which actually require ten bedrooms, so lots of bedrooms are a luxury which is put into houses which also have other luxuries like being detached ect. On the other hand, if your house just has lots of bedrooms, it doesn't cost much more. My parents had a five bedroom semi because they had four children and actually found it a bit difficult to find a buyer for. The most practical thing if you wanted to have a polygamous household would be to build new or adapt housing. Knocking through two houses could be a good option, or building an extension. You could also then adapt the houses in other ways to suit polygamous families better. I read a while ago that you could tell that polygamy was declining in a particular country because most new housing only had one kitchen. Even women willing to share a husband tended to be unwilling to share a kitchen.