Detached ADUs

AN ACT Relating to establishing limitations on detached accessory 1
dwelling units outside urban growth areas; amending RCW 36.70A.696; 2
and adding a new section to chapter 36.70A RCW. 3
BE IT ENACTED BY THE LEGISLATURE OF THE STATE OF WASHINGTON:4
NEW SECTION. Sec. 1. A new section is added to chapter 36.70A 5
RCW to read as follows: 6
(1) Counties may allow detached accessory dwelling units outside 7
of urban growth areas if the county meets the requirements in 8
subsection (2) of this section, and such detached accessory dwelling 9
units are subject to development regulations that include the 10
following limitations: 11
(a) No parcel may have more than one attached or detached 12
accessory dwelling unit; 13
(b) The detached accessory dwelling unit is subject to the water 14
supply requirements of RCW 19.27.097 and any groundwater mitigation 15
requirements adopted by the county or department of ecology;16
(c) The applicant must provide documentation that the existing or 17
proposed sewage or septic system is capable of handling the 18
additional demand placed upon it by the detached accessory dwelling 19
unit; 20
S-0528.3
SENATE BILL 5470
State of Washington 69th Legislature 2025 Regular Session
By Senators Goehner, Bateman, Chapman, Frame, Gildon, Nobles, and
Saldaña
Read first time 01/23/25. Referred to Committee on Housing.
p. 1 SB 5470
(d) The gross floor area of the detached accessory dwelling unit 1
may not exceed the gross floor area of what could be authorized by 2
the county as an expansion of the primary dwelling to create an 3
attached accessory dwelling unit and, in no case, exceed 1,296 square 4
feet. Floor areas exclude garages, porches, and unfinished basements;5
(e) The detached accessory dwelling unit must utilize the same 6
driveway as the principal unit; 7
(f) The detached accessory dwelling unit must be sited within 150 8
feet of the principal unit; 9
(g) The detached accessory dwelling unit may be the existing 10
principal unit if the existing principal unit meets the requirements 11
of this subsection, is a single-family dwelling unit, and a new 12
principal unit that is a single-family dwelling unit is constructed 13
on the same parcel; 14
(h) A parcel may not be subdivided for the purposes of avoiding 15
the limits on development regulations described in this subsection;16
(i) If the detached accessory dwelling unit is offered as a 17
short-term rental as defined in RCW 36.70A.696, the primary unit must 18
be owner-occupied; and 19
(j) The combined water withdrawal for the detached accessory 20
dwelling unit, primary unit, and any other domestic uses on the 21
parcel must not exceed the use limitations in RCW 90.44.050 for 22
domestic use. 23
(2) Counties may allow detached accessory dwelling units outside 24
of urban growth areas if the county takes the following actions to 25
account for detached accessory dwelling unit development:26
(a) Track and report to the department annually the number of 27
detached accessory dwelling unit permits completed;28
(b) Update the county's comprehensive land use plan during the 29
county's next required review and all subsequent reviews according to 30
RCW 36.70A.130(5)(b) to properly account for the number of detached 31
accessory dwelling units completed since the effective date of this 32
section and the projected development over the next 20-year planning 33
period utilizing the data collected and reported in (a) of this 34
subsection within the overall underlying density outside of urban 35
growth areas; 36
(c) Future amendments to the county's comprehensive land use plan 37
accounting for actual and projected detached accessory dwelling units 38
within the overall underlying density outside of urban growth areas 39
may not occur more than once every five years. 40
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(3) Subsection (1) of this section is in addition to other county 1
authority enumerated in this chapter and does not: 2
(a) Affect or modify the validity of any county ordinance 3
authorizing accessory dwelling units adopted prior to the effective 4
date of this section; 5
(b) Exclude other means of authorizing accessory dwelling units 6
in urban or rural areas, if consistent with this section; or7
(c) Exclude other innovative techniques under RCW 8
36.70A.070(5)(b), 36.70A.090, or 36.70A.177, if consistent with this 9
section. 10
Sec. 2. RCW 36.70A.696 and 2023 c 334 s 2 are each amended to 11
read as follows: 12
The definitions in this section apply throughout RCW 36.70A.697, 13
36.70A.698, 36.70A.680, ((and)) 36.70A.681, and section 1 of this act 14
unless the context clearly requires otherwise. 15
(1) "Accessory dwelling unit" means a dwelling unit located on 16
the same lot as a single-family housing unit, duplex, triplex, 17
townhome, or other housing unit. 18
(2) "Attached accessory dwelling unit" means an accessory 19
dwelling unit located within or attached to a single-family housing 20
unit, duplex, triplex, townhome, or other housing unit.21
(3) "City" means any city, code city, and town located in a 22
county planning under RCW 36.70A.040. 23
(4) "County" means any county planning under RCW 36.70A.040.24
(5) "Detached accessory dwelling unit" means an accessory 25
dwelling unit that consists partly or entirely of a building that is 26
separate and detached from a single-family housing unit, duplex, 27
triplex, townhome, or other housing unit and is on the same property.28
(6) "Dwelling unit" means a residential living unit that provides 29
complete independent living facilities for one or more persons and 30
that includes permanent provisions for living, sleeping, eating, 31
cooking, and sanitation. 32
(7) "Gross floor area" means the interior habitable area of a 33
dwelling unit including basements and attics but not including a 34
garage or accessory structure. 35
(8) "Major transit stop" means: 36
(a) A stop on a high capacity transportation system funded or 37
expanded under the provisions of chapter 81.104 RCW;38
(b) Commuter rail stops; 39
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(c) Stops on rail or fixed guideway systems, including 1
transitways; 2
(d) Stops on bus rapid transit routes or routes that run on high 3
occupancy vehicle lanes; or 4
(e) Stops for a bus or other transit mode providing actual fixed 5
route service at intervals of at least fifteen minutes for at least 6
five hours during the peak hours of operation on weekdays.7
(9) "Owner" means any person who has at least 50 percent 8
ownership in a property on which an accessory dwelling unit is 9
located. 10
(10) "Principal unit" means the single-family housing unit, 11
duplex, triplex, townhome, or other housing unit located on the same 12
lot as an accessory dwelling unit. 13
(11) "Short-term rental" means a lodging use, that is not a hotel 14
or motel or bed and breakfast, in which a dwelling unit, or portion 15
thereof, is offered or provided to a guest by a short-term rental 16
operator for a fee for fewer than 30 consecutive nights.17
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