Several companies to card stock modern buildings, but Fat Dragon Games advertise their buildings as being fold flat and as they have a sale on until the end of December I jumped in now with a purchase of the base city set and the first expansion.
Here are a couple of the basic builds for you to see.
The main building construction is very basic 4 walls with tabs on the bottom to glue to a chosen base (if you don't want to fold flat). The roof is constructed as a sort of hat which fits over the outside of the walls. The included instructions say to add a tab across the the top if you intend on doing the folding flat design (which I do). I had thought that I would need to strength the walls, especially on the 3 story block (I'm using 210gsm card) but the tabs top and bottom sit perpendicular to the walls themselves and really add rigidity to the structure once the rood segment is on.