Fees are determined based on the amount of IA on a parcel. The median IA of all Single Family Residential (SFR) parcels in Wellesley is 3,105 ft2. One equivalent residential unit (ERU) is represented by this median value of 3,105 ft2. The total ERUs in town was determined in combination with estimated revenue requirements to develop a cost per ERU. The fee does not consider the percentage of imperviousness on a parcel. SFR parcels will be billed a tiered flat fee and Non Single Family Residential (NSFR) properties will be billed proportionally to their impervious area as shown below. For proportional fees, a given parcel is divided by the impervious area in one ERU (3,105 ft2) to determine a rounded, whole number of ERUs on the parcel.