Here's my suggestion:
- Create a new home
- Choose Plan > Add level menu item
- Select room creation tool and draw the shape you want
- Choose 3D view > Modify 3D view menu item and select the transparency level you want with Walls transparency slider
- Choose 3D view > Export to OBJ format and confirm the option Export selection when prompted
- Import in your main home the exported OBJ file with Furniture > Import furniture menu item