The three building sections: left, middle and right wall were originally meant to go on top of the base, then I would place the sidewalk or pavement sections in front.
However, it became clear once I placed them that it would cause problems since the base of the right wall had a protruding edge (door, see photo above) that would mean awkwardly cutting around it and exposing a strange discrepancy between surfaces.
So I opted to have them on top of the same surface:
Once the layer was primed, I glued it on. Then I attached the buildings to it and secured them with backing supports.
Next: finishing the exterior details