In those instances where I need to get under buildings, such as if I have electronic modules residing there (easier access than trying to get my 70-year old bod under the layout!), I simply let them sit there. In those instances where I want them secured to the layout, I either mould a foundation using plaster, then remove the building while the plaster dries, lay down some of that now-unavailable AMI tar roadbed as a foundation, or a drop of Walther's GOO at each corner, or some other rubber-based cement that will hold the building, but comes up fairly easily without damaging the material of which the building is made.