Use a designer for your project. You will get a more accurate cost of the project.

Ask the builder what his process is for the selection of building materials, does he allocate allowances for items such as countertops, cabinets, lighting, doors etc. If so does he have a designer selection coordinator he works with or will you be responsible to go to the different vendor locations for assistance?
What level quality does he use for the material and product allowances? Is it a good/better/best approach? Will you have a cost allocation sheet to help you stay on track and avoid going over budget?
A strong plan at the start makes for an incredibly successful project in the end.