Ok. My down and dirty house was ugly so I'm not posting it but I did come up with some nice concepts and discoveries. First of all, if you try to stack foundation blocks it doesn't work. Anyway I haven't found a way to make it work without using cheats and that's not allowed in legacy. However, you can build an adequate "house" by using the roof on the ground! You take the "on", off, and just start drawing your roof. I used the half roof and wound up with a basic lean to shape that was quite large and very tall. At the short end the space is unusable as it is not tall enough for a Sim to stand in. However the rest of the space is quite usable and the cost of that "room" is ZERO. Roofs are free, and the game thinks you are just putting on a roof. I added a door, put in flooring and had all the basics for a legacy starter house. Not attractive, mind you, although I didn't fool with it much and I bet I could have made it prettier. Later on in life this "building" can be demolished or used as a shed or studio.
Now I'm wondering if you could still stack foundation blocks by using the terrain tool to raise the ground, add a block and remove the terrain under it. Much like you do for floating houses only using terrain. Then build up the terrain again, just low enough to add a foundation block UNDER the first one, rinse and repeat till you have a column or series of columns to form a wall. Seems like an awful lot of work for very little return though.
I'm still experimenting with how high you can go too. So my trial house also was built on top of a step pyramid and was quite tall. If I didn't have challenges to finish I would be having so much fun with this!