Here is my suggestion on how to get an estimate on costs. First, find a plan online that you like well enough. You can check out houseplans.com or dreamhomesource.com. The plan can be close, but it doesn’t have to be perfect. Be careful about making extensive changes. This means you may have to look through lots of plans. Send the plan to startbuild.com. For $30 They will give you a breakdown for material and labor for your zip code for about 20 different building categories. Once you get their report they have drop downs where you can select the grade of materials you want to use and it will update costs. My experience is they seem reasonably accurate. You can spot check a few things if you want. I wouldn’t necessarily buy the online plan, by the way. But this is a way to get a reasonably accurate cost on a plan close to what you would build.
The second approach is to go on to Zillow and find some new construction around you. Again look for a house as similar to your online plan as possible. Write down the asking price for the house. Then find a lot around you and again write down the asking price. Now subtract the lot price from the house price you wrote down and multiple by .85. If the startbuild.com plan you used and the house you found on zillow are similar square footage divide both the startbuild cost by the square footage and the zillow house minus lot and builder cost by the square footage of the zillow house. If those two different costs per square foot are fairly close +/- $20 per square foot you will have a pretty good estimate of the cost per square foot in your area for a size house you estimated. Realize that it is pretty common for a larger house to have a lower cost per square foot. High cost areas like kitchens and bathrooms tend to be fairly constant. Additional floor area from lager rooms doesn’t add as much proportionally as do the “expensive” rooms.
If you plan to do a lot of the labor yourself, you can transfer the information to a Googlesheet spreadsheet and customize it to remove labor costs for your sweat equity (if you plan on doing any of your own labor) and get even closer to a final estimate.