If you are seriously considering a pit, aside from building regs, a few other things to consider are drainage of water that may seep through the walls, ventilation of gases that sink into the pit and jacking pads along the sides of the pit. Also any lighting in the pit should be intrinsically safe to prevent ignition of any fumes. The pit also needs to be longer than the car to allow free and unrestricted entry and exit into the pit with the car in position. Finally you have to consider means to protect anyone coming into your garage from falling into the pit.
At Doncaster railway works because of the risks associated with pits and water ingress we filled them in and now use jacks to lift rolling stock for working on the underside.