I've been thinking up and collecting ideas on how to scratch build modern buildings (my last scratch-build was almost 2 years ago) so I've been a bit anxious to try out some of these techniques!
Clear Acrylic Sheets and Sintra:
The key I've found with getting good, straight edges with acrylic is that you don't actually want to 'cut' all the way through...that seems to either lead to a devastating crack or an uneven break. The trick is to score it just enough so that you can snap the piece along your score line against a solid table edge or something. Its not easy, and is the most aggravating part of the project!
Windows and Exterior:
I'm using my new Kato buildings (shown in the top photo) as a guide for floor height, which is about 22.5mm or 7/8". I decided to use Evergreen Plastics 349: .040x.250 (1mm x 6.3mm) as the exterior material to go between the floors. At 6.3mm width, they are just wide enough to cover the 5mm foam core sections which will be the interior floors.
I'm not sure if this is making any sense, so maybe the pictures below will make it more clear what I'm talking about!
Given that I have my 7/8" floor height, I went into Intaglio (basic drawing program) and created a grid for the exterior window frames. The vertical distances are 10mm, which gives a pretty good indication of windows. I printed this drawing onto transparency paper using a laser printer and - wallah! - I have my window frames! This is a much easier way of adding window details than trying to cut precise little pieces of styrene, etc... Its a technique I read about a couple of years ago in Railway Modeller.
For the styrene strips themselves, rather than making the angle two pieces, I decided to try and keep the pieces together and try to avoid a problematic seam on the corner. I scored the back of the styrene, painted, and then used a bit of heat to make the bends.
Ground Floor and Base:
For the ground floor/lobby, I decided I wanted it recessed from the rest of the structure (which seems like a typical design), but nearly double height. Again, I used the 'ground floor' height of the adjacent Kato building so that they would consistent with each other. The base itself is a sheet of styrene (Sintra is too thick for a floor), which I then applied some styrene "L" pieces to hold the plastic windows (which will be the support for the entire building) solidly in place.
Finally, I also started working on the interior floors. These are mostly cut from foam core, with additional vertical sections to which I glued a homemade interior wall patterns. A picture does a better job of explaining this: