@MarMarcin Do you mean houses that you've downloaded, or houses that you've built yourself? Any file that is saved to the in-game bin—households, houses, community lots—will also appear in the bin when you start a new save. These files are located in Library inside your game folder. If you want to create a new game folder, just copy over the Library folder, and your content will show up in the game.
If you've built a house in your save that you want to keep, all you have to do is go to Edit Town and save it to the bin.