I'm creating a tilemap city and trying to figure out the most efficient way to create unique building scenes. The trick is, I need to maintain a sort of 2D, almost-top-down perspective, which is hard to do with buildings or large objects that span multiple tiles.

I've tried doing three buildings at a time, and mixing and matching the base layer and colors, like this:

This creates a weird overlapping effect, and also doesn't seem that efficient from a production standpoint. But it was the best way to have shadows appear correctly on the neighboring buildings.

I'm wondering if modular buildings would be the way to go? That way I can mix and match any set of buildings together as tiles:

I guess I would have to risk some perspective and shadowing to get the buildings to align correctly.

What sort of authoring process could I use to allow me to create a variety of buildings (or other objects) that maintain this perspective while spanning multiple tiles worth of screen space? Would you recommend creating blank buildings, and then affixing art overlays as necessary to make the buildings unique? Or should they be directly part of the building tile (for example, create a separate tileset of buildings signs and colorings)?