#12 for bath gfci also.
And depending on code cycle, #12 for some garage receps.
For what it's worth, I don't think a residential Sparky is all that efficient running under slabs and then tying it in later. Do what you do best. Drill it out and pull it in Romex.
I wired 8000+ sf custom and semi-custom homes for several years. I'm guessing a 12,000 sf spec home, you'll probably have 16,000 feet of wire.
That means you will probably spend $5,000 more than you anticipated. But honestly, I would think if you are at $10 per square foot on a spec home you are probably okay.