For buildings with entryways at curb level that accommodate ramps, stairs or lifts to dwelling units that are elevated above curb level on the first story of the building, up to 100 square feet of such entryways may be exempted from the definition of floor area for each foot of difference between the floor level of such dwelling units and curb level. However, no more than a maximum of 500 square feet of floor space may be exempted from the definition of floor area for each building.