a house should cost a lot more than a private room, no?
To keep things simple I think the actual space shouldn't be significantly bigger then the best sleeping option available in the region. So in a city you would own a room in a building, in a rural you'd own a small cottage.
I would do the inverse and limit it to one house per adventurer. What commoner has two houses? This would also reinforces the social status aspect ("You live in my house at my sufferance, you better do what I tell you.")
Yeomen would have two houses possibly more.
