Best approach to low-poly house modeling?

[Stiglr]

Seems to me that bottom ledge is a separate surface, not the same as the floor to the house. You should be able to select it alone and delete it without affecting the floor.

Isn't the floor the much darker grey color under the wall (or is that the viewport background?

In any case, the floor should be its own object, such that any operations done to a wall won't affect the floor one way or the other.

Just cut the floor away as a separate object; select it in Surface Mode and do Cut Away Object.

As to your Boolean question, yes. If you don't set it up to do otherwise, AC3D creates a new object in the exact same place, but if you're doing a boolean cut, you can't see the result for the overlapping original object.