A builder builds what an architect design. Or, a design/build firm does both. Never let a builder design your house.

Location and complexity of the home are the biggest determinants of cost. "Square footage cost" is the roughest of unreliable price estimates. A custom build can cost between $100 a square foot for a square metal building somewhere in Nowhere, Arkansas, to $900 a square foot in San Francisco, California. Buying existing is cheaper than building. Look at what is available locally that would be similar to what you'd want to end up with. Then add a 20-50% custom premium on top of that price.

Look at the Find a Pro for a local architect to design something. You'll need the lot first as the architect will design something that fits the land.