June 27, 2024District's Policy on ADUs


Accessory Dwelling Units (ADUs) offer a way to include smaller, affordable homes in 
established neighborhoods. These give homeowners the flexibility to share independent living areas with family members and others while maintaining privacy, including but not limited to allowing seniors to age in place as they require more care, helping extended families to be near one another, and providing rental income. ADUs help to address housing shortages by increasing supply in single-family neighborhoods and providing an affordable place to rent. The benefits that ADUs offer have made them increasingly popular. 

The purpose of this Accessory Dwelling Unit (ADU) Policy is to formally address ADU fees and charges and provide additional requirements for these dwellings as they pertain to water improvements. To the extent the requirements contained in this Policy differ from or conflict with the District Standards and Details, this document shall control. 

An ADU in King County Water District No. 90 (District) must be lawful and permitted 
through the agency having land use permitting authority. For the District, this would be the City of Renton or King County. It is not the purpose of this policy to regulate land use within the authority of other local jurisdictions but rather to provide rules and regulations limited to the jurisdictional authority provided to Water and Sewer Districts pursuant to Chapter 57.08 RCW. This policy and the rules and regulations contained herein should be read within that context.


Accessory Dwelling Unit (ADU) - An ADU is defined as an accessory dwelling unit that is either attached to the primary single-family dwelling unit (AADU) or is a detached 
accessory dwelling unit (DADU) that is located on the same residential parcel as the 
primary residence. An ADU is a habitable single-family dwelling unit that provides basic living, sleeping, eating, cooking, and sanitation requirements.

Attached Accessory Dwelling Unit (AADU) – An AADU is an ADU that is physically
attached to the primary single-family dwelling unit.

Detached Accessory Dwelling Unit (DADU) – A DADU is an ADU that consists partly or entirely of a building that is separate and physically detached from the primary single family dwelling unit. Examples would include, but not be limited to, tiny houses, backyard cottages, or converted outbuildings.

Equivalent Residential Unit (ERU) - The term used for the typical amount of water a single family residence uses over the course of a day. 

Single-Family Dwelling Unit – A detached single-family structure containing one dwelling unit, commonly known as a house.


The meter serving an ADU (DADU or AADU) may be shared with the primary single-family residence or separately serve the ADU.

Existing Meter – ADUs can be served by the existing service line and meter from the District’s water main. Customers with ADUs will be charged a double-use rate. (Two times the Base Rate and two times the Consumption Tiers.) Property Owners with an ADU can purchase a “deduct” meter if they desire to know and/or bill the water for the ADU separately. The deduct meter will be purchased by, and belong to, the Property Owner. The District will not perform maintenance, meter readings, or billing on a deduct meter. 

Every ADU created after November 30, 2023 must be served by at least a 1-inch service line from the water main to the meter, along with a 1-inch meter to serve an ADU and the Primary Residence. A larger service line and/or meter is only required if necessary to serve the ADU adequately. If the Property Owner proposes to serve the ADU with an existing meter and the District determines that the meter is insufficient to do so, the Property Owner must upsize the meter. 

Separate Service and Meter -  Pursuant to HB1337 passed on April 6, 2023, “A city or county may not prohibit the sale or other conveyance of a condominium unit independently of a principal unit solely on the grounds that the condominium unit was originally built as an accessory dwelling unit.”

HB 1337 allows Property Owners to sell an ADU as a separate lot. The primary residence and the ADU cannot be sold separately unless the City of Renton or King County allows the parcel to be split. For this reason, ADUs built after April 2023 should have a separate meter connection for the ADU and will pay the full installation and fees to have a separate meter installed. The service line from the Meter to the ADU is the property owner’s responsibility. The connection charges below (#4) are due when a separate service connection is installed. Property Owners who do not have a separate service connection will have a notice placed on their property title clearly stating that a separate service connection is required to be installed before an ADU is sold as a separate dwelling. Any legal fees and recording costs will be charged to the property owner. The charge for a future separate service line and 
meter will be based on the District’s Schedule of Charges applicable at the time the service is installed. 


ADUs (DADU & AADU) at or under 1,000 square feet will be assessed a General Facility Charge (GFC) that is based on one Equivalent Residential Units (ERU) for water where applicable. 

The cost of the meter installation, either a “Meter Drop” or a “Full Install,” must be paid to the District before the new meter is installed. Meter Drop and Full Install rates are included in the District’s current Schedule of Charges. Additionally, The Seattle Public Utilities Facility Charge (aka Conservation Fee) must be paid at the current single-family rate. All fees are due at the time of meter purchase and/or meter upsize.


The District supports the State’s housing goals as defined in Engrossed House Bill 
1337. The goal of the bill is to “expand housing options by easing barriers to the 
construction and use of accessory dwelling units.” Additionally, the bill states, “The 
legislature intends to promote and encourage the creation of accessory dwelling 
units as a means to address the need for additional affordable housing.”
Section 5 of HB 1337 states, “Cities and Counties may only offer such reduced or 
deferred fees, deferred taxes, waivers, or other incentives for the development or 
construction of accessory dwelling units if: 

1) The units are located within an urban growth area; and

2) The units are subject to a program adopted by the city or county with 
effective binding commitments or covenants that the units will be 
primarily used for long-term housing consistent with the public purpose 
for this authorization.”

To support the State’s goals of creating more low-income housing, ADUs meeting 
these conditions are eligible for a 15% reduction on the District’s General Facility 
Charge. All other fees (permits, meter installation, paving, etc.) are the same rate 
as a single-family home. 

Once approval of an ADU is provided by King County or the City of Renton (i.e.,
construction permit), the Property Owner must complete a Water Availability Application in advance of construction to ensure sufficient water is available to the property. Additionally, the property owner is responsible for informing the District when the ADU is ready to be connected to the original service line/meter or when a separate meter and service line are ready to be installed. District staff will install the service line from the main to the meter along with a new meter. The property owner is responsible for installing the service line from the meter to the ADU.

Bimonthly billing will begin for an ADU once a new meter or new connection is made.
Additional fees for permits, inspections, road restoration, and additional staff time will be charged to the property owner. All known fees must be paid in advance of the meter installation. Occasionally, charges for additional staff time and road repair are not known until after the work is completed. Additional charges are due 30 days after being invoiced.

Failure to inform the District that an ADU has been connected to the District Water System will not stop bi-monthly base rate and consumption charges being applied retroactively to the date of connection.