Best option will be to order an interior project, if you have architectural layout ready and you are completely certain that you are really going to build a house.
As quicker you make up your mind, what exactly you would like to see in your house, as more exactly you will be able to get overall cost citation. It is rather hard to forecast finishing costs without an interior project. It is also easer for builders to work upon a ready project rather than during construction process to change and rebuild existing solutions. A common mistake is to call builders and only then to start developing an interior project. Please keep in mind that development of an interior project on the average takes 2 months, also provisions shall be made for time to order floor materials, plumbing, kitchen, etc. So, the best option is to develop an interior project first and then start talking with builders.