just because a building is old does not necessarily mean it is a "qualified historic structure." to qualify, it must be designated on a national, state or local register (so for your example a local designation would qualify - it does not have to be a state designation). and the SHBC technically does not apply since if there is no work being done, it is the ADA that would require making the restroom accessible. however, if the building ends up being designated, that does not excuse it from ADA compliance. it must still be made accessible unless to do so would irreparably damage historic fabric.
as sharon says this issue is often misinterpreted by building officials, architects and building owners. and there is no such thing as grandfathering, even for designated historic resources.