They are talking about house with a foundation to it. You can do it, but you have to have the room to make a basement, same principal applies, say you want to put stairs towards one side of living room, move any objects that might be in the way. Delete a section of foundation, put stairs on any side, go down a level and build your basement, find a spot where you cut out your foundation, place stairs, there will be a platform where your first level is so you need to carpet the area. You can build a wall with a door, sort of like a stair well or decorate it with those partitions. It takes up alot of space to do this, I have done it before on an existing house, but it has to be a big house. The only problem, you have to go down two levels to get to the basement, but that does not bother me in the least.