Sewer lines that are to be extended shall have the same hydraulic capacity and be constructed on the same grade line as the existing sewers unless the Board determines that a reduction of capacity is justified. Sewers shall be sized with capacity for the contiguous service area which is defined as the undeveloped and/or unsewered land capable of a gravity connection to the proposed sanitary sewer or lift station, unless it is shown that the contiguous area may be equally served by an alternate existing sewer.