Houses have been done before. Enough time that we are good at those estimates. Once you have built the same house a dozen times you can estimate to within a couple hundred dollars. Contractors do this all the time for spec houses. Spec meaning that they are building it and the planning to sell it once it is complete. I know several contractors who do this all the time. They have never had someone order a spec house and come in anywhere close to the estimates. Everyone goes over the light fixture budget ($600) by at least $3000. Similar for the kitchen and bath fixtures. It isn't unheard of to double the cost of the house on seemingly minor upgrades.