Foundations and Decks
This page describes the Foundation Tool.
The Foundation Tool
Many people insist that the insides of their foundation walls match the outsides for more realism.
Foundations can be removed by pressing the Ctrl key and retracing your foundation. Foundations much be clear of objects to be deleted. If you want to delete everything, try the sledgehammer tool instead.
Foundations need not appear as a solid block under a house. Look at some of the smaller houses that come with the game. You will see foundations just dotted here and there under the house in small rectangles. This can make the house have the appearance of being built up off of the ground with a crawl space underneath.
To get a good idea of some of the foundation possibilities, look at some of the pre-made lots.
They will give you some ideas of what is possible.