Great question, tho I'm not aware of any OFFSET command such as you would find in any drafting software - that's not what BB is meant for, but I agree it would be nice, I encounter lots of times that I would use that as well.
The only way I know how to do this is manually. Well, unless your foundation is a square, then you can group-scale the square up or down. But if any lines are rectangular, etc., it wouldn't scale it up equally on all sides.
