The maximum area to be serviced by the proposed main sewer line and its ultimate branches and laterals shall be determined by the city based on local drainage conditions. If the area to be serviced by the developer is less than the maximum area to be serviced by the proposed main sewer line and its ultimate laterals, such main sewer line shall be designed, engineered and constructed to serve such maximum area. Should other parties in the service area desire to join with the developer in constructing the proposed main sewer line, the agreement between the developer and the city shall be considered as including those additional parties. (1976 Code § 11-4-3)