No matter how much money you have, you are going to have to design your own house within a budget. You will need to know how many dollars per square foot you can afford.
Visiting your bank's mortgage department is the best way to figure how much house you can afford to build. They will ask you financial and job related questions which you will need to answer truthfully and honestly. The numbers they will give you are tied to the interest rate at that time which may go up or down by the time you actually want to borrow the money. However, you can use those numbers to get a ball park figure which will help you know how much you can afford when you start to design your own house.
Once you have an idea of how much you can afford, you will want to start making decisions about your home design. You could also start to call some contractors to see what their rates are but it is generally best to do this after you have an actual home design plan to show them. This all assumes you already have the plot of land that you want to build on. If you have yet to buy the land, then you should work on that first.