I've just finished two houses for use in 40mm
Sharp Practice games. I scratch-built the walls from foamcore board, and used the roofs from two 1/72 Italerei kitset houses I had on hand for smaller scale games (the roofs of these kits are removable, so no problem to switch them for whatever scale game I'm playing).
Here's a couple of 40mm Perry riflemen entering a two-storey house:

And here's its accompanying barn/farm cottage (it has a large open wagon door on the back - that's why you can see the background through the window!):

Here's a rear-view of both buildings, with a couple of 40mm Perry French to give an idea of their size:

Here are the buildings with the roofs removed, so you can see the simple interiors:

The top floor of the bigger house is also removeable, to allow for fights involving men on both storeys:

And here are the original 1/72 Italerei kitsets that I used the roofs from (ignore the police armoured cars in the foreground!):

I used the same overall technique to build and paint these houses as described in an article I wrote a couple of years ago for the Kapiti Fusiliers website on making 25mm houses:
http://www.fusiliers.net/wargaming6.htm