RTS building placement collision problem C#

I think you should keep a reference count for how many active collisions there are. For example, if you would collider with one object, then another object, then uncollide the last object it would set bm.collided as false, which probably would allow placing the object. Also you are not checking for the case when you stop colliding with the floor.

Something like OnCollisionEnter >> ++collisions, OnCollisionExit >> bm.collided = (--collisions) == 0, and add the guards for the floor if you need to.