For example: basements start at ground level, not the level of the bottom floor, so the first basement in a building with a foundation is 5 modular stairs levels deep, not 4. This meant i could not fit the basement stairs in one house, but the game happily allowed me to connect a 4 section modular stair at right angles to the top 1 section modular stair landing. I tested it and sims use the stairs and end up in the basement even though it would require crawling through a 1/4 height area under the fireplace to get to the second flight of stairs.
I ran into the basement/foundation issue as well, I couldn't fit the stairs in the house I was building and instead placed them outside partially under a porch. It would have caused my sims to have to bend over backward to use them but they seem to have no problem with it. I'll be accounting for that from now on and I might try your solution in that house if I can fit it.I'm glad you guys like the layout so far. I have only structurally completed a couple of the 10x10 lots. The are 2 bed 1.5 bath houses with no basement. I might add that in. I would love to see your screenshots, and as soon as my schedule gives me a chance, I will finish one of the houses and upload it. I am planning on making all of the 10x10 lots similar, like in a neighborhood.
I made some of the 10X10 lots into city-style 3 story lofts. The second and third floors are the house with the first floor being an art gallery, adventure display, library, etc. Non-resident sims don't use them or anything but I found it interesting and if there's an expansion like Open For Business it would work really well.