Possible Effects of New Accessory Dwelling Units on Water Distribution Systems

Washington Suburban Sanitary Commission (WSSC Water) is among the largest water and wastewater utilities in the United States, serving approximately 1.9 million residents in Montgomery and Prince George's County, Maryland. WSSC Water's Planning Division evaluates the capacity of the entire water transmission system through Master Planning, while WSSC Water's Development Services Division (DSD) performs a Hydraulic Planning Analysis (HPA) for new development and re-development projects. An HPA will be required when an applicant/developer requires a change of service category and/or requests a new water or sewer service for a property or set of properties, which need extension of the existing WSSC water system. In addition, Montgomery County Planning Subdivision regulations require that the applicant who submits a proposed Preliminary Plan of Subdivision must submit verification from WSSC Water that they have applied for an HPA. However, Montgomery County in July 2019 amended its zoning laws to make it easier to build attached and detached Accessory Dwelling Units (ADUs), which do not require ADU applicants to apply for an HPA. This paper will discuss the effects of ADUs on the existing water distribution system, especially in areas where the water system is already stressed.