Chapter 20.55
ACCESSORY DWELLING UNITS (ADUS)
Sections:
20.55.030 General requirements.
20.55.050 Coverage requirements.
20.55.120 Permitting and enforcement.
20.55.020 Definitions.
For the purposes of this chapter, certain words and terms shall be interpreted as follows:
“Accessory dwelling unit (ADU)” means a dwelling unit located on the same lot as a principal residential dwelling unit.
“Attached accessory dwelling unit (A-ADU)” means an accessory dwelling unit that is attached by at least one common wall to the principal dwelling unit.
“Detached accessory dwelling unit (D-ADU)” means an accessory dwelling unit that is completely structurally independent from the principal dwelling unit.
“Dwelling unit” means a residential living unit that provides complete independent living facilities for one or more persons and that includes permanent provisions for living, sleeping, eating, cooking, and sanitation.
“Principal dwelling unit (PDU)” means a residential living unit that is the primary dwelling unit on a lot that also houses accessory dwelling units. (Ord. 1832 § 1, 2025)
20.55.030 General requirements.
All ADUs shall meet and comply with the following regulations:
(1) ADU Construction. A maximum of two ADUs may be constructed on a single lot. Pairs of ADUs may be constructed in the following combinations:
(A) Two A-ADUs, such as units in a basement or addition;
(B) One A-ADU and one D-ADU; or
(C) Two D-ADUs, which may be comprised of either one or two detached structures, pursuant to the lot coverage and setback requirements as per Sections 20.55.050 and 20.55.070.
(2) Existing Structures. An existing structure, including attached or detached garages, may be converted into an ADU. If converting a garage to an ADU, the required minimum number of off-street parking spaces must be met and maintained, including pursuant to this chapter and the provisions of Chapter 20.56. An existing structure that does not conform to current setback or lot coverage standards may be converted to an ADU. However, the conversion must comply with all other applicable regulations, including maximum ADU size limits.
(3) Sewer Restriction. ADUs are prohibited on properties which are not connected to and served by public sewer.
(4) Size and Scale. The ADU maximum size shall be no more than one thousand square feet and minimum size shall be no less than seven hundred square feet.
(5) Flood Damage Prevention. ADU location and construction must comply with Chapter 14.20, Flood Damage Prevention.
(6) Entrances. The following are regulations regarding the design and placement of ADU entrances:
(A) When the entrance for both a PDU and an A-ADU on the same property are oriented to the street, the entrance for the larger unit shall be designed to appear as a primary entrance and the entrance for the smaller unit shall be designed to appear as a secondary entrance and shall appear incidental in nature.
(B) Both D-ADU and PDU entrances should be oriented toward the street, where practical.
(7) Addressing. ADUs and PDUs on the same property shall have the same street address number, but be delineated using a letter A, B, and/or C at the end of the number.
(8) Utilities. D-ADUs shall have separate utility connections/hookups and meters from the PDU and the owner of the D-ADU shall be held responsible for paying all applicable connection/hookup fees.
(9) Design. ADUs should be designed to include exterior finish materials, trim, and roof pitch which are similar in type, size, and placement material to that of the PDU, including two-foot eave overhang and proper foundational support, and following all adopted building codes. The ADU should also be designed to be harmonious in nature to that of the surrounding neighborhood. The design requirements for ADUs under this subsection shall not be interpreted to be more restrictive than those for the PDU on the same property. (Ord. 1832 § 1, 2025)
20.55.040 Lot size.
The minimum lot size of a property containing an ADU shall be consistent with the minimum lot size requirement of the respective zone. (Ord. 1832 § 1, 2025)
20.55.050 Coverage requirements.
A property which contains an ADU must meet the minimum lot coverage requirement of the zone so that no more than thirty-five percent of the lot area shall be covered by buildings. (Ord. 1832 § 1, 2025)
20.55.060 Height.
A D-ADU may not exceed a roof height of thirty-five feet when measured from the bottom of the foundation, or that of the PDU, whichever is lower. An A-ADU may not exceed the roof height of the PDU, unless the A-ADU is converted from an existing structure. (Ord. 1832 § 1, 2025)
20.55.070 Setbacks.
Due to the city routinely plowing and maintaining public alleys, all ADUs must comply with the setback requirements of the existing zone in which the ADU is located. If no setback requirements are outlined in the applicable zoning chapter, refer to the setback requirements outlined in Chapter 20.32. (Ord. 1832 § 1, 2025)
20.55.080 Parking.
(1) Unless otherwise required by the city in conformance with the provisions of RCW 36.70A.681(2)(b) or other applicable law, the following parking requirements shall apply to ADUs:
(A) If the property upon which the ADU is located is six thousand square feet or less before any zero lot line subdivisions or lot splits, there shall be one off-street parking space per unit as a condition of permitting development of the ADU; and
(B) If the property upon which the ADU is located is greater than six thousand square feet before any zero lot line subdivisions or lot splits, there shall be two off-street parking spaces per unit as a condition of permitting development of the ADU.
(2) If the property abuts a public alley, ADU parking shall be accessed from the alley, where practical. (Ord. 1832 § 1, 2025)
20.55.100 Condominium sale.
The sale or other conveyance of a condominium unit independently of a principal unit shall not be prohibited solely on the grounds that the condominium unit was originally built as an accessory dwelling unit. No sale or other conveyance of a condominium unit independently of a principal unit shall be permitted unless it complies with, and meets the requirements of, all other applicable zoning and codes.
Prior to the sale or conveyance of a condominium unit which was originally built as an accessory dwelling unit, all water, sewer, and electric utility connections must be separated from the principal dwelling unit. (Ord. 1832 § 1, 2025)
20.55.110 Landscaping.
Properties which include an ADU must provide open space and landscaping that are useful for both the ADU and the PDU. Landscaping shall provide for the privacy and screening of adjacent properties. The open space and landscaping requirements for ADUs under this subsection shall not be interpreted as more restrictive than those for the PDU on the same property. The owner of the property shall be given sole responsibility of maintaining the landscaping of the whole property, pursuant to Chapter 20.57, unless a mutual agreement can be met between the residents of the PDU and ADU. This agreement must be submitted to the building official in addition to a proposed landscaping plan prior to issuance of an occupancy certificate. (Ord. 1832 § 1, 2025)
20.55.120 Permitting and enforcement.
(1) Application. The property owner shall apply for an ADU permit with the city building official. This application shall be processed as a Class II permit and the requirements outlined for said permit class in Chapter 20.08.
(2) Applicable Codes. The ADU shall comply with all construction codes set forth in Title 14 and the latest adopted International Residential Code and any other applicable codes, except as provided in this chapter.
(3) Inspection. After receipt of a complete application and prior to approval of an ADU, the city building official shall inspect the property to confirm that all applicable requirements of this code and other codes are met.
(4) Permit. Upon compliance of the provisions of this section, a permit for an ADU will be issued.
(5) Elimination/Expiration. Elimination of an ADU may be accomplished by the owner, recording a certificate with the Whatcom County auditor and Sumas building official stating that the ADU no longer exists on the property, contingent on verification by the Sumas building official.
(6) Variance. Variances to this chapter shall require variance approval as outlined in Chapter 20.64.
(7) Violations. Any violation of any provision hereof is a violation under Chapter 20.75. (Ord. 1832 § 1, 2025)