This "new" law has nothing to do with washing the car or not. It has to do with water coming/running on the street/public places.
You can wash your car (on private property) as long as the water does not run off on the street. However my neighbor washes their cars everyday and all the run-off water gets into my garage porch drive way. Until today he has not stopped doing it so I assume they have not been hit with a fine yet.So I do not think they are strict in enforcing this law. Assume they are much stricter for flats and inner city properties.