It's not too difficult to stay close to a budget if you can avoid making changes and lock in the price of materials.
It's the "might as well's" and "while your at it's" that cost a lot of money.
Yea all the "might as well's" and "while your at it" are already in the cost right now. The original price was $46,800. The cost after going through the design phase jumped to ~$59,000. The contractor forecasts his material needs 6 months in advance and prepays for it with his supplier. He said this saves him and me like $3,000 on a job like mine.