You shouldn't do one big displacement. It makes the ground look very weird and your displacement paint look awful. I would say if you made all those buildings with correct measures (64, 128, 256, 512...) you should use 256x256/512x512, power of three displacements to create basements.
Also create some kind of support structure right in the middle of the destroyed wall. It doesn't only stop the house from collapsing it also makes it more harder for zombies to rush in. That just looks too OP for an entrance anyway.