Per square foot never gets you anywhere.

Talk to some builders whos work you like. See what they say.

As you probably have figured out - you get what you pay for.

I live in NC so not terribly different COL. I have built for $110, $150, and $180.

Obviously when an architect and builder come over and give you a slick presentation, they aren't the cheapest option. But you aren't going to be happy with the cheap ones either. I am sure that you can find something in the middle. But getting someone with a reasonable rep to build for $400k is going to be a small challenge.

Have you considered modular?

Is your lot expensive to work on? Do you have any idea? Flat, access to utilities etc?

You'll need to get actual bids to see what your market is charging to build a house. There are too many variables for there to be a 'rule of thumb'. I will say that we built a house in Texas (Dallas area) which I'm guessing to be about the same or possibly a bit higher COL area compared to Atlanta. The housing/building market is crazy hot here over the past few years so getting stuff inexpensively was really hard. And crappy sub-contractors were unfortunately more common that good ones.

My DH and I acted as the GC/builder for our own house in part because we like stuff like this and in larger part to save quite a bit of money. Acting as our own GC, actively scouring the internet for the best possible buys from building materials to finishes, and doing a significant portion of the build and finish out ourselves (DIY), we are barely keeping it at about $105-$110/sq. foot. Our finish choices were fairly mid to high end.....all hardwood, quartz, custom cabinets, etc. But also used IKEA in a bathroom and two Home Depot vanities. Light fixtures ranged from ones found on Craigslist to Restoration Hardware.

So to expect to get a turn-key house, fence, landscaping, site prep, driveway for $100-$120/ft is probably not realistic. If you get close, you'd be picking the cheapest builder and using less expensive materials, like laminate counters, carpet, vinyl windows, etc. Absolutely nothing wrong with choosing those things but that's where you're at with that budget. And like you've said, your house shape would have to be simple. I prefer simple myself, but many others like nested gables, stone and brick, lots of dormers, etc. Those type of things add to the cost.

I owned the land and still my cost to build came in at 190/sq ft. I am not in the suburbs of a major city either, but 8 miles from a 800 population town. I would think it costs more to build near a major metropolis due to supply/demand. I don't know what you would get for 90/sq ft.

Find out what kind of finishes 90/sq ft gets you and can you live with it? Pay special attention to windows and framing because you want those to be the best you can afford, I doubt 90/sq ft pricing includes quality windows but I am in a different area.

The only way to know is ask your GCs and see what you get for the prices. Nothing wrong with a basic house with basic upgrades, my aunt has lived in a small, but functional home with basic finishes all her life and still loves it. It's economical to replace things as they wear out, easier to clean and care for and it has been paid off for years giving her the freedom to enjoy life instead of being tied to house payments.

Sometimes I wish I had gone that route!

Hopefully you've read some of the threads on cost to build. A couple of things you'll see is that simple is good (fewer corners mean simpler and less expensive foundations and roofs) and that a smaller home will often have a higher cost / sq foot because that small home will still need the expensive spaces (such as kitchens and bathrooms) and will have less of the 'cheap' spaces (large rooms without plumbing, fixtures, etc.) You'll also read, as you do above, that there are too many factors to estimate and cost per sq ft is not a reliable way to estimate. Something like windows (number, brand, style) can change the cost by tens of thousands without changing the square footage.