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CITY OF RENTON Ι PERMIT SERVICES
ACCESSORY DWELLING UNITS
WHAT IS AN ACCESSORY DWELLING UNIT (ADU)?
Commonly known as a mother-in-law unit, backyard cottage, or granny flat, an Accessory Dwelling Units (ADU) is an
independent dwelling that shares a lot with a single-family home. Renton Municipal Code (RMC) defines an ADU as
“An independent subordinate dwelling unit that is located on the same lot, but not within a single-family dwelling.
This may include units over detached garages.”
ADUs are intended to create affordable, flexible housing opportunities that take advantage of the City’s existing
infrastructure while addressing the need for increased housing choices that reflect changing lifestyles and
environmental concerns.
HOW IS MAXIMUM ADU SIZE DETERMINED?
The maximum ADU size for each property depends on several factors, including the size of the lot, the area of the
primary residence, and the impervious surface coverage standards of its zone.
MAXIMUM ADU SIZE*
LOT AREA MAXIMUM ADU SIZE 1
3,000 sq. ft. or less 600 sq. ft.
3,001—4,999 sq. ft. 700 sq. ft.
5,000—6,999 sq. ft. 900 sq. ft.
7,000—8,999 sq. ft. 800 sq. ft.
Greater than 9,000 sq. ft. 1,000 sq. ft.
1 The total gross floor area of the ADU shall not exceed that allowed based on lot size or 75% of the gross floor area of
the primary residence, whichever is smaller.
When calculating the maximum ADU size allowed on your lot, be careful to consider the maximum impervious surface
area for your zone (see table on next page). Impervious surface areas are humanmade “hard areas” (non-vegetated
surfaces that prevents or slows the entry of precipitation, i.e., snow and rainwater, into the soil.
Common impervious surfaces include, but are not limited to, roofs, walkways, patios, driveways, parking lots, storage
areas, areas that are paved, graveled or made of packed or oiled earthen materials, or other surfaces that similarly
impede the natural infiltration of surface water or stormwater. Please note that ADUs do not count toward maximum
building coverage/lot coverage.
WHERE ARE ADUs ALLOWED?
Most residential zones in Renton allow you the opportunity to build an onsite ADU, including properties zoned RC, R-
1, R-4, R-6, R-8, R-10, and R-14. ADUs must be a minimum of 6 feet from the primary residence.
ADUs are not allowed between the primary structure and the street unless approved in the Conditional Use Permit
process. The minimum rear yard setback in all zones is 5 feet. The minimum side yard setback varies by zone and is
included in the Development Standards for Residential Development at the end of this document or visit RMC 4-2-
110C, Development Standards for Residential Development (Accessory Dwelling Units), for more information.
rentonwa.gov/permitservices | planningcustomerservice@rentonwa.gov | 425-430-7294 10/23/2023 Page 2 of 4
ACCESSORY DWELLING UNITS
MAXIMUM IMPERVIOUS SURFACE AREA
RESIDENTIAL ZONING DESIGNATION IMPERVIOUS SURFACE AREA (%)
RC 15%
R-1 25%
R-4 50%
R-6 55%
R-8 65%
R-10 70%
R-14 80%
IS OWNER OCCUPANCY REQURED?
Owner occupancy is required unless the applicant applies for and is granted a Conditional Use Permit. Proposals must
meet the criteria outlined in RMC 4-9-030D and meet the decision criteria for ADU owner occupancy exemption in RMC
4-9-030H. All other applicants will be required to submit an owner occupancy covenant that will be recorded in the real
estate records of the King County Recorder’s Office.
ARE PERMITS REQUIRED?
Yes, the applicant will need to apply for permits from the Building Division in order to ensure that you meet the
requirements of the International Residential Code and other safety-related requirements. Building plans are required
at the time of application submittal. Additionally, a Conditional Use Permit is required in each of the following scenarios:
1. The proposal includes locating the ADU between the primary residence and a street; or
2. The property owner does not plan to live on site. (Only proposals that meet the criteria of RMC 4-9-030D and RMC
4-9-030H are eligible for non-owner occupancy.)
DO ADUs REQUIRE ADDITIONAL PARKING?
In most cases, ADUs will require a minimum of one additional parking space and a maximum of two is allowed. However,
ADUs located within a ¼ mile of a mass transit facility are exempt from off-street parking requirements. If the primary
structure on a site where an ADU is proposed does not meet the City’s minimum parking standards, one additional off-
street parking space is required.
WHAT OTHER REQUIREMENTS ARE NECESSARY FOR ADU APPROVAL?
• Architectural Character to be Consistent: The ADU must be consistent with the architectural character of the primary
structure and shall be required to meet the ADU Residential Design Standards. If the primary dwelling associated
with the proposed ADU does not comply with the adopted architectural detailing standards, the primary structure
shall be brought to compliance prior to the issuance of ADU building permits. Please see RMC 4-2-116B.2 for more
information.
• Owner Occupancy Affidavit Required: The property owner shall file an affidavit affirming that the owner will
occupy the primary dwelling or the Accessory Dwelling Unit.
• Notice on Title Required: Prior to the issuance of building permits, the owner shall record a notice on the property
title. The notice shall bear the notarized signature of all property owners listed on the property title and include the
legal description of the property, a copy of the approved plans, and the applicable City restrictions and conditions.
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ACCESSORY DWELLING UNITS
DOES THE CITY ALLOW GARAGE TO ADU CONVERSIONS?
Conversion of accessory buildings, including detached garages is permitted. If the existing structure was built prior to
January 1, 2020, the ADU will be exempt from select development standards, such as maximum ADU size, location, and
setbacks. If the project proposal is for an accessory building to ADU conversion, it will still be required to meet applicable
building and energy code.
HOW DO I APPLY?
If you are interested in ADU construction, please do the following:
1. Fill out the Engineering Improvement Determination form on our website. This request is intended to help applicants
better understand what types of site improvements may be applicable to the property when preparing for an ADU
permit application. Please remember to include staff’s Engineering Improvement Determination analysis with other
submittal documents at the time of application.
2. Download the application, submittal requirements, and electronic file standards. Please read the submittal
requirements carefully so that you know what is required for project submittal.
3. Contact Permit Services at (425) 430-7215 or permittech@rentonwa.gov to request an FTP link to upload your plan
submittal. If we find that the application is incomplete, we will ask that you resubmit once you have all the required
documents. Please note: Some fees will be collected at the intake appointment.
4 . Pick up your permits! Your project manager will notify you when your permit is approved and ready to be printed.
There may be additional items needed, so please review the email for any further instructions. This email will also
include the invoice for the remaining balance; please print the invoice and bring it to the Finance Department on the
1st floor or pay online.
Prior to submitting your building permit application to Development Services, we strongly encourage you to meet with
a Planner during walk-in customer service hours (8:00 a.m. to 4:00 p.m.) for an informal site plan review. You may also
submit your site plan for review to planningcustomerservice@rentonwa.gov.
DEVELOPMENT STANDARDS FOR RESIDENTIAL DEVELOPMENT (ACCESSORY DWELLING UNITS)
MAXIMUM NUMBER AND SIZE
General: The lot coverage of accessory dwelling units shall not be calculated towards maximum building/lot coverage.
RC, R-1, R-4, R-6, R-8, R-10,
and R-14
1 ADU is permitted per legal lot in association with a detached single -family home.
Unit size shall be determined by lot size and the size of the primary structure; the total
gross floor area of the ADU shall not exceed the size stated in the Maximum Unit Size
section of this table or 75% of the total gross floor area of the primary structure.
Maximum Unit Size:
Lot Area Maximum ADU Size 2, 3
≤ 3,000 sq. ft. 600 sq. ft.
3,001-4,999 sq. ft. 700 sq. ft.
5,000-6,999 sq. ft. 800 sq. ft.
7,000-8,999 sq. ft. 900 sq. ft.
≥ 9,000 sq. ft. 1,000 sq. ft.
rentonwa.gov/permitservices | planningcustomerservice@rentonwa.gov | 425-430-7294 10/23/2023 Page 4 of 4
ACCESSORY DWELLING UNITS
MAXIMUM WALL PLATE HEIGHT 3
RC, R-1, R-4, R-6, R-8,
R-10 and R-14
ADUs are subject to the maximum wall plate height of RMC 4-2-110A, and associated
conditions and shall not be taller than the primary structure. Additional ADU height allowances
may be permitted upon application and approval of a modification pursuant to RMC 4-9-250.
4
LOCATION 3
RC, R-1, R-4, R-6, R-8,
R-10 and R-14
ADUs shall be located at least 6 ft. from any residential structure.
MINIMUM SETBACKS 3
Front Yard and Secondary Front Yard:
RC, R-1, R-4, R-6, R-8,
R-10 and R-14
The ADU shall be setback an additional 5 ft. parallel to and measured from the front façade of
the primary structure and shall comply with the setbacks applied to the primary structure, as
identified in RMC 4-2-110A, Development Standards for Residential Zoning Designations.
Side Yard:
RC and R-1 25 ft.
R-4, R-6, and R-8 5 ft.
R-10 and R-14 4 ft.
Rear Yard:
RC, R-1, R-4, R-6, R-8,
R-10 and R-14
5 ft. When located within 10 ft. of the rear property line, at least 25% of the lineal length of
the rear yard shall remain unoccupied from accessory dwellings, except when the rear
property line abuts an alley.
Clear Vision Area:
RC, R-1, R-4, R-6, R-8,
R-10 and R-14
In no case shall a structure over 42 in. in height intrude into the 20 ft. clear vision area
defined in RMC 4-11-030.
CRITICAL AREAS
RC, R-1, R-4, R-6, R-8,
R-10 and R-14
See RMC 4-3-050, Critical Areas Regulations, and 4-3-090, Shoreline Master Program
Regulations.
2 The square foot calculation shall not include porches or exterior stairs. Garages attached to accessory dwellings shall be
included in the square foot calculation, except for when the accessory dwelling is located above a garage.
3 Conversion of accessory buildings to ADUs shall be exempt from the relevant development regulations if the accessory
building was constructed prior to January 1, 2020. However, modifications made to accessory buildings after January 1,
2020 that would increase the nonconformance of the proposed conversion are ineligible from such exemptions.
4 ADUs built using City produced pre-approved ADU base plans may exceed the wall plate height of the primary structure
by four feet (4’) and may be allowed an additional height allowance upon application and approval of a modification
pursuant to RMC 4-9-250. Applicant produced ADU plans seeking to exceed the wall plate height of the primary structure
may be allowed upon application and approval of a modification.