One thing to keep in mind when considering dwellings: birds need a lot of room to spread their wings sufficiently to fly. So if they don't have stairs, they'll have large open spaces with large gaps in the floor/ceiling, as they would want to be able to jump down and glide to the next level or fly up through the opening without banging their wings on anything. They'd also need swooping room on the upper end so they can fly up and land safely.

They probably build big. Where primitive humans emphasized small buildings with walls to enclose, the harpy primitive society likely has roofs as the first, most important thing. They provide shelter. Then floors, to extend the landing area, though that would likely be a "catch-net" first, for storing things that don't balance well on perches. Walls would come later. Their predators largely come from below and above.

And in all cases, again, they build for big and expansive. That is, lots of open airspace for ease of maneuvering. To them, open air space is as important as open floor space is to people. Moreso, since they can't stop to delicately maneuver unless they're awkewardly hopping on floors or perches.