New idea for terrain holes in interiors

There is already an existing feature that terrain will not render inside a portalised interior, correct? So if you stick a house halfway through a hil, you don't have the hillside sloping through your kitchen. The problem is, you still collide with the hill side, correct? That's why you 'set empty', so you don't collide. But then your hole is pretty regularly shaped, and may not line up nicely with the house.
The fundamental problem is colliding with the terrain. So how about just checking each shape - if they're inside an interior, disable terrain collisions. That way you can have tunnels or buildings nicely intersecting terrain without needing tpo tidy up holes, and not be walking into invisible walls all the time.
Worry is that it might be expensive to check whether each and every object is inside all the time. Maybe trigger areas - when you're in a trigger area which denotes proximity to an interior, then you start checking.