While most bike racks are adjustable they don't offer a universal fit. They are just simply too many different shaped vehicles for that to be possible.
If you're planning on buying a rear rack you'll need to be aware of the shape of your car’s rear end. Different rear racks are designed to work best with saloons/sedans, hatchbacks or people carriers. If you are in any doubt, log on to the rack manufacturer’s website where you should find a vehicle checker, showing which products fit your motor.
Likewise a specific roof rack may not be compatible with your current roof bars. These come in two shapes: traditional square-profile and more modern oval, aero bars. The oval bars are designed to cut through the air, reduce drag, and in the process cause less wind noise. Just make sure when you pick your roof rack it will work with whichever shape roof bars you have fitted.