Some custom home builders start with a stock plan and then add on addition, modifications and customize the design. Other custom home builders will work from scratch using the services of a architect or other design professional. It is somewhat less expensive to go with modifications rather than a completely new plan, but also has more restrictions as to final design.
Before deciding to build a custom home it is critical to do a lot of research. You will need to understand any building regulations or zoning criteria for the area, as well as research exactly what you want In your home. The internet offers lots of opportunity to look around at options available, but remember not all may work in your area.