A building contractor is a tradesman who would take full control of the development and he would employ directly the other trades people. There would normally be a set price for the works to be carried out at the commencement of the job.
It may be a case that the price is based on the size of the build with a cost per sq. meter.
This would include to a certain finish or stage, be it painted or tiled or even completed to include flooring, hanging of light fittings and the installation of appliances.