Just to clarify, it's 400' AGL. And if you have a 800' building within 400', you can legally fly 1200' AGL above and around the building, as long as you're within 400' of that building. If you're climbing a mountain lets say...... the RC will allow you to go up to 1640' above takeoff point, but you are obligated to stay within 400' of land in any direction. This is where VLOS is important, since the craft cannot determine AGL, they can only determine AAHP (altitude above home point), today. Having a really strong strobe light helps to keep an eye on it.
I really look forward to the day drones have down facing laser range finders incorporated.
This 400' above a structure is only if you have Part 107 remote pilot certificate. It does not apply to hobbyists. Hobbyists do not have a legally enforced altitude limit, except in relation to regulated airspace. The 400' altitude limit is a "suggestion" by the AMA, the "community based organization" who's rules most hobbyists try to follow.