I was being partially felicitous, but it does bring up a potential loophole to be honest And made me wonder.
The 400 ceiling is not agl, it’s related to your takeoff point (which usually is all, but not always). If you go to the roof of a 400 ft building and take off, you’re allowed to go up to 800ft, and you must then remain 400ft max away from other buildings or the ground. So if the building is the only building in the area and you travel 400ft horizontally away from the building, then you have to descend to 400ft agl.
Likewise with full size aircraft, they must remain minimum 500ft from other objects, so flying over the same location they could come no lower than 900ft agl