@Zeke just looking at your location under your logo you are in Long Beach, CA. I've not lived or worked in AL but pretty sure those prices are going to be considerably less than your location. Looks like Enterprise, AL is the major town in Coffee County, AL and I quickly pulled these for reference.
Example 1 = Existing structure, 3,919sf @ $424,000 = $108/sf
2627 Highway 51, Enterprise AL, is a Single Family home that contains 3895 sq ft and was built in 1962.It contains 2 bathrooms.This home last sold for $410,000 in November 2022. The Zestimate for this Single Family is $482,000, which has increased by $2,232 in the last 30 days.The Rent...
www.zillow.com
Example 2 = New construction, 2,250sf @ $375,000 = $167/sf
203 Cypress Ln, Enterprise AL, is a Single Family home that contains 2220 sq ft.It contains 4 bedrooms and 3 bathrooms. The Zestimate for this Single Family is $340,100, which has increased by $1,048 in the last 30 days.The Rent Zestimate for this Single Family is $2,310/mo, which has...
www.zillow.com
Doubtful either example nails their situation perfectly, especially considering the whole 5 minutes of research I put into the effort. Also, the numbers above include land costs. Their numbers to build would exclude that since they already own/financed their land separately. Hard to say for sure as finish levels determine so much, but I'm guessing they will be $110-125/sf finished out for both the house & shop combined.
I do agree that if OP can put the garage portion underground, and build out the living space above, that will help decrease construction costs. Also, thermal effects will be amazing. Had a family relative do this and his garage feels like he has a/c during the summer and a heater during the winter.
I might add that I have done a custom built home. Well, I designed and then interviewed & hired a builder for the day-to-day operations. For about 2 years I did lots of research and walked through a plethora of homes gaining ideas and learning what I liked and wanted. In this interim I used AutoCAD to create, revise, revise & revise some more the floorplans, LOL. I can visualize things pretty well, but one of the things that continued to amaze me was the house shrunk and grew throughout the building process. Laying out the position of the house on the land, it seemed way too small. During framing it began to feel large enough. Then came sheetrock and it shrunk again.
All visual effects mind you. Just my point is that going on-site and using flags or string line may or may not be beneficial. I'm lucky that I have AutoCAD at my disposal and have for most my career. There aren't too many projects I embark on where I don't draw up something to help get my head clear. Here is a recent sampling where I laid out my workbenches and was considering some different lighting options -- won't bore with those details.