Chicago's storm water ordinance requires special calculations if you are planning to construct a sunken driveway for a lower level garage access. There have been numerous cases where the street flooded over the top of the sidewalk and the overflow for the entire block was into the basement of an unsuspecting home owner that ended up with six feet of water in their basement. The ordinance now requires that the engineer prepare a plan to determine the overflow path of the storm water in the gutter in the event that the catch basin nearest the property is blocked. The question is : will the sewer overflow down the street to another block or will the new driveway become the overflow point? This requires topographic survey of the gutter along the subject block and then down to the next cross street until an overflow point is found. Calculations are required for the amount of runoff that would be expected in the gutter and the depth of the gutter flow. This is then compared to the grade at the back of the sidewalk to make sure the driveway can be constructed safely. Builders should not automatically assume that a sunken driveway will work in every location.