I've been foraying into stairs-- these are the kit stair sides:

With steps an inch high, this simply wasn't going to work for the mice. I started putting together a winding staircase based on 1/2"-high steps and 5/8"-treads, from foam board and matboard, to see how it would look in the space:

Those pieces that extend into the room are just base pieces that would get cut off, but regardless as much as I liked how they turned out, this staircase simply took up too much space in the room. Back to the drawing board-- tried 1/2"-tall steps along the wall and they ended up at the front door, so compromised with 5/8"-tall steps:

Relocated the doorway (4" tall!), and I think this will work. The card on the floor shows how wide they'll be. I would prefer the steps to be 1/2"-tall, but I think this is minimally intrusive on the rest of the room.

Anyway, I had to do this so I could figure out where the stairwell hole would be in the second floor. Here is everything back in rough dry-fit:

The reason that the front door opening is so tall is that it will have a transom window. The bay walls are two layers of matboard glued together; I cut out the windows in each layer separately b/c cutting two layers together would've been too tough!

I am deviating from the original house's roofline by keeping that right-hand-side roof tall-- it may look a little funny but I want a third floor, and if I lower it to the left-hand-side roof height, there won't be room enough for a third floor.

Now that everything's cut, including doors and windows, I've taken it apart and primed. Last weekend I went to get paints and papers:

The paint at the top is Valspar perfect pint interior latex satin in "Botanical Bliss", and I even found square "diamonds" at Michael's for the left gable. I was disappointed with the cardstock selection at HL and Michael's-- the florals are in paper, whereas I am thinking that some of these rooms will do better with a thicker cardstock wallpaper due to the amount of wall patching that I've done (therefore increasing the surface roughness of the walls). I am looking at this site to see if there are some printed cardstocks that catch my eye.