The most popular way to build a car is to strip away a used car's platform and assemble parts on top of it, which sometimes allows builders to classify cars as modifications even if most parts are new. Builders must still create the engine, the drivetrain and the control interface.
The first step of building a car is deciding on an engine and drive train. Builders generally need to weld supporting structures into space, which may require a license. The control interfaces often come next to ensure that they work with the engine and drivetrain. The body of the car is often installed near the end of the process since it typically involves metal fabrication or installing carbon fiber, which can be expensive to modify if the car's dimensions need to be changed.
One way to simplify the process is to purchase a kit car. Caterham Cars, for example, sells kits for drivers to assemble. Even if a builder ultimately wants to create a completely custom car, constructing a car from a kit can teach some of the basics of building cars. Because they contain so many parts, cars are difficult to build; many custom cars are built by small companies and sold at high margins. In addition, most companies that build cars use at least some parts from other manufacturers. For example, many purchase engines instead of designing new ones.