Cost estimating is a complex process. Early "ballpark" estimates are not accurate because there is so much guesswork involved. As the design phases unfold the details will emerge to narrow in on a realistic budget. An experienced design profession can help to guide your decisions as your go and keep your costs under control. Likewise an experienced contractor on your team can join in meetings and add cost advice in the early design phases.
Sustainable Sedona recommends the “front load approach” where you make all your decisions about your house inside and out ahead of construction during design. Make sure your final set of construction drawings is detailed completely to eliminate the guessing game. The “front load approach” will be the best decision you can make to contain the cost of your project.