This is a small but growing modular range of mdf buildings from Sarissa Precision. Link to City Block products here.
I haven't tried/purchased any of these yet, but I may well consider doing so in the future. The buildings offer a variety of frontage options and can be stacked with additional floors, so a lot of customisation options before any other considerations.
For a 3-storey building (single extra floor) they look to be around £20 GBP for the smaller footprint building (155 mm x 105 mm footprint; top picture), or £28 for the larger footprint (200 mm x 155 mm; middle). I think the larger footprint building would be just a little smaller than the Ziterdes Apartment buildings I already have. I can imagine that they would be able to use the core designs for stuff like a factory etc. in the future.
There is a residential City Block coming (right) with fencing and steps, and apparently a warehouse too, so the visual profile of a mix of buildings should be nicely varied. The only downside for me is the integral pavement pieces, since that won't fit with my city boards. But if playing on table of a black or very dark grey it would give instant pavement and therefore road layout.