Bothell's code has caught up with Washington's ADU law and, on one point that matters a great deal, it goes further than the state requires. Two accessory dwelling units are allowed per lot, at up to 1,200 sq ft each, with no owner-occupancy requirement — and BMC 12.14.135(C) grants one ADU even on lots designated with critical areas or their buffers, which is exactly the class of lot that RCW 36.70A.681(4) strips of every state protection. A detached ADU is exempt from the rule capping accessory buildings at five percent of lot area, may stand 30 feet tall (33 feet over an existing accessory structure), takes the same rear yard setback as the house, and may sit at the alley setback where the lot adjoins one. Bothell also wrote the state's impact fee cap into its own code and opened an amnesty route for ADUs built without a permit. Two things to check yourself: the code host blocks automated access, so the text verified here is the version current through Ordinance 2464 of September 16, 2025; and Council eliminated parking minimums for new development on July 8, 2025, which may have superseded the one-space-per-ADU rule that was last verifiable in BMC 12.16.030.
Zoning & Size Requirements [1][2][3][4][5][6]
Yes — Bothell allows ADUs. A detached ADU can typically be up to 1,200 sq ft under current Washington and Bothell zoning rules.
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| Allowed Zones | Permitted (P) in every residential zone — R-C, R-L1, R-L2, R-M1, R-M2, R-M3, R-M4, R-MHP and R-AC — and not permitted in the OP, NB, CB, GC and LI zones (Bothell Municipal Code 12.06.140(A), Residential Uses table). Development condition 4 states that "Accessory dwelling units are permitted when added to, created within, or detached from a primary dwelling unit(s) subject to the restrictions in BMC 12.14.135" (BMC 12.06.140(B)(4)). Bothell renamed its residential zones as part of its 2024-2025 code updates; older documents still use the R 40,000 / R 9,600 / R 8,400 / R 7,200 / R 5,400 / R 4,000 / R 2,800 labels. RCW 36.70A.681(1)(c) independently requires two ADUs in all zoning districts inside an urban growth area that allow single-family homes. Confirm your parcel's zone with Bothell Community Development at 425-806-6400 |
| Max Size (Detached) | 1,200 sq ft. "An accessory dwelling unit may be developed up to 1,200 square feet" (Bothell Municipal Code 12.14.135(D)(1)) — 200 sq ft above the 1,000 sq ft floor state law guarantees (RCW 36.70A.681(1)(f)). Conversion of an existing space may exceed 1,200 sq ft as approved by the community development director, provided it does not exceed 40 percent of the primary dwelling unit's gross floor area. Critically, "Detached accessory dwelling units are exempt from the provisions of BMC 12.14.130 that limit the size of accessory buildings to five percent of the lot area" (BMC 12.14.135(D)(2)) — without that exemption a 1,200 sq ft detached unit would need a 24,000 sq ft lot |
| Max Size (Attached) | Same 1,200 sq ft cap, with the same conversion allowance: an ADU created by converting existing space may exceed 1,200 sq ft as approved by the community development director, so long as it stays within 40 percent of the primary dwelling unit's gross floor area (Bothell Municipal Code 12.14.135(D)(1)) |
| Max Size (JADU) | Not a category in Washington — Bothell's code has no junior ADU designation. The nearest equivalent is an ADU created within the walls of the existing house, which is treated as an ordinary attached ADU and benefits from the conversion allowance at BMC 12.14.135(D)(1) |
| Max Height | 30 feet for a detached ADU, or 33 feet for a unit built over an existing accessory structure (Bothell Municipal Code 12.14.135(E)). That sits below the 35-foot principal-building height allowed in the R-C, R-L1, R-L2, R-M1 and R-M2 zones (45 feet in R-M3, 55 feet in R-M4 — BMC 12.14.030(A)), but it is comfortably above the 24-foot floor that RCW 36.70A.681(1)(g) protects, so it stands. An attached ADU is part of the principal building and takes the zone's height limit |
| Max Units | 2 ADUs per lot — "Two accessory dwelling units are allowed per lot" (Bothell Municipal Code 12.14.135(A)), and the Residential Development Standards table carries a "Maximum ADUs per lot" row of 2 for the R-C, R-L1, R-L2 and R-M1 zones (BMC 12.14.030(A)). Two exceptions cut the allowance to one: "One accessory dwelling unit in any configuration is allowed on a lot developed with middle housing" (BMC 12.14.135(B)), and "One accessory dwelling unit is allowed on lots designated with critical areas or their buffers" (BMC 12.14.135(C)). The second is a gain, not a loss — RCW 36.70A.681(4) exempts critical-area lots from every state ADU protection, so on those lots Bothell's own code is the only thing granting you an ADU at all |
Setback Requirements [1][2][3][4][5][6]
Bothell requires a rear setback of about 35 ft for a detached ADU, and one off-street parking space is required. Owner-occupancy is not required — you can rent it out without living on the property.
| Rear Setback | The same rear yard setback as the house: "Within all residential zone classifications, rear yard setbacks for accessory buildings and structures, other than detached accessory dwelling units, shall be five feet. Detached accessory dwelling units shall maintain the same rear yard setback as primary structures, except for those accessory dwelling units located on an alley, in which case the setbacks for structures adjoining alleys in BMC 12.14.060 shall apply" (Bothell Municipal Code 12.14.070(B)). The zone minimums where there is no alley are 35 feet in R-C, 15 feet in R-L1 and R-L2, and 10 feet in R-M1 through R-M4; where the lot adjoins an alley the rear setback drops to 0 feet, or 3 feet for garage doors (BMC 12.14.030(A)). Note the odd result: an ordinary garden shed gets a 5-foot rear setback while a detached ADU in R-C must sit 35 feet back. State law bars setbacks for ADUs more restrictive than those for principal units (RCW 36.70A.681(1)(h)), and Bothell's rule matches the principal unit exactly |
| Side Setback | 5 feet in every residential zone — R-C, R-L1, R-L2 and R-M1 through R-M4 all carry a 5-foot minimum side yard setback for the principal building (Bothell Municipal Code 12.14.030(A)), and a detached ADU takes the principal-structure standards. Setbacks are measured under BMC 12.14.050 through 12.14.100; projections allowed into a setback are listed at BMC 12.14.090 |
| Front Setback | 15 feet in the R-C, R-L1 and R-L2 zones and 10 feet in R-M1 through R-M4, except that any garage door must be set back 20 feet (Bothell Municipal Code 12.14.030(A)). BMC 12.14.030(B)(4)(b) adds that garages, carports and other structures intended to house vehicles must be set back at least 20 feet from streets and access tracts. The area within the front yard setback may not contain hard surface except driveways, walkways and permitted projections |
| Parking Required | One off-street space per ADU as last verifiable, waived on transit or trail proximity — and possibly now zero. BMC 12.16.030(A) lists "Dwelling units, accessory — 1 per unit, unless waived per BMC 12.16.110(F)", and BMC 12.16.110(F) states the ADU parking requirement does not apply where the ADU is within one-half mile walking distance of a major or frequent transit stop, or within one-half mile walking distance of pedestrian access to a regional trail that provides a continuous two-way paved connection to employment centers or similar activity centers and amenities (Ord. 2415, 2024) — the trail waiver is Bothell's own addition and reaches much of the Sammamish River and Burke-Gilman corridor. Since then, on July 8, 2025, the City Council "approved the elimination of required parking minimums for new developments," citing SB 5184. Whether that repeal reached the single-family ADU row could not be verified for this entry because Bothell's code host blocks automated access — confirm with Bothell Community Development at 425-806-6400 before you give up a parking space. Either way, RCW 36.70A.681(2)(a) caps what Bothell may require at zero within half a mile walking distance of a major transit stop, one space per unit on lots under 6,000 sq ft, and two per unit on larger lots |
| Owner Occupancy | Not required. BMC 12.14.135 contains no owner-occupancy condition, no rental restriction and no recorded covenant. Bothell could not impose one in any event: "The city or county may not require the owner of a lot on which there is an accessory dwelling unit to reside in or occupy the accessory dwelling unit or another housing unit on the same lot" (RCW 36.70A.681(1)(b)) |
| Min Lot Size | No ADU-specific minimum, and state law requires a city to allow an ADU on any lot that meets the minimum lot size for the principal unit (RCW 36.70A.681(1)(e)). The zone minimums for the lot itself are 40,000 sq ft in R-C, 6,000 sq ft in R-L1 and 3,600 sq ft in R-L2, with no minimum in R-M1 through R-M4 or R-AC (Bothell Municipal Code 12.14.030(A)). Lot size averaging applies in R-L1 and R-L2 land divisions (BMC 12.14.030(B)(2)(b)). The practical size constraint on a detached ADU is hard surface coverage — 35 percent of the lot in R-C, 55 percent in R-L1, 60 percent in R-L2, 70 percent in R-M1 — not lot area, because BMC 12.14.135(D)(2) exempts detached ADUs from the five-percent accessory building cap |
Permit Process & Fees [7][8]
An ADU permit in Bothell goes through a standard permit review. Confirm with Bothell Development Services at 425-806-6400 — no ADU review timeline is published on the city's ADU submittal checklist. Budget separately for the water and sewer availability certificate, which the checklist puts at up to two weeks where the City of Bothell is the provider. Permit fees: At submittal (2026 fees, from the city's ADU checklist): a plan check fee based on project valuation, a Single Family Planning Review fee of $231.00, and a 5 percent technology fee. At permit issuance: the permit fee, plumbing, mechanical and state fees; a Residential Site Review and Inspection fee of $834.00 where you are constructing an addition or a new building; a stormwater facility charge if you are adding square footage; and the 5 percent technology fee on everything except the state fee, impact fees and facility charges. For a new detached ADU the plumbing and mechanical permit fees are each 8 percent of the building permit fee, 16 percent in total; for a remodel or addition creating an ADU they are a base fee of $82.00 plus a per-fixture fee, most residential fixtures running $27.00 each.
| Approval Process | A building permit, applied for online through MyBuildingPermit.com: select Building, then Single Family Residential, then Activity Type, then Accessory Dwelling Unit (City of Bothell Development Services, Accessory Dwelling Unit submittal checklist, form 3f, updated December 2025). There is no conditional use permit, no hearing and no recorded covenant. An assigned address letter from the City must be obtained first (form 54), and a water and sewer availability certificate plus a fire flow availability letter are required where you are adding square footage — if Bothell is your water provider, apply for Water Sewer Availability (form 36) first and allow up to two weeks. Where an ADU already exists without an approved application, the owner must apply to the city for approval, and where it was built without a building permit the city will require a building inspection to confirm the structure is sound and meets BMC 12.14.135 and BMC Title 20; the ADU application fee covers that inspection (BMC 12.14.135(F)). State law bars the city from requiring public street improvements as a condition of permitting an ADU (RCW 36.70A.681(1)(l)). Bothell City Hall, 18415 101st Ave NE: 425-806-6400 |
| Review Timeline | Confirm with Bothell Development Services at 425-806-6400 — no ADU review timeline is published on the city's ADU submittal checklist. Budget separately for the water and sewer availability certificate, which the checklist puts at up to two weeks where the City of Bothell is the provider |
| Permit Fees | At submittal (2026 fees, from the city's ADU checklist): a plan check fee based on project valuation, a Single Family Planning Review fee of $231.00, and a 5 percent technology fee. At permit issuance: the permit fee, plumbing, mechanical and state fees; a Residential Site Review and Inspection fee of $834.00 where you are constructing an addition or a new building; a stormwater facility charge if you are adding square footage; and the 5 percent technology fee on everything except the state fee, impact fees and facility charges. For a new detached ADU the plumbing and mechanical permit fees are each 8 percent of the building permit fee, 16 percent in total; for a remodel or addition creating an ADU they are a base fee of $82.00 plus a per-fixture fee, most residential fixtures running $27.00 each |
| Impact Fees | Traffic, fire and park impact fees apply to a new dwelling unit, and Bothell has written the state cap into its own code: "The city will not assess impact fees on the construction of accessory dwelling units that are greater than 50 percent of the impact fees that would be imposed on the principal unit" (Bothell Municipal Code 12.14.135(G)), tracking RCW 36.70A.681(1)(a). Current dollar amounts are not published on the ADU checklist — ask Development Services at 425-806-6400 for the impact fee form, and hold them to the 50 percent cap |
| Plan Check | Plan check fee is based on project valuation. Plans must be no larger than 24 by 36 inches, drawn to scale, and the site plan must show hard surface coverage calculations under BMC 12.14.140(B), distances from existing and proposed structures to property lines, footing and roof drain location and discharge method, and any environmentally sensitive areas on the site, adjacent to it, or within 200 feet — wetlands, steep slopes, top and toe of slope, rivers and streams. That last item is not busywork: critical areas are what decide whether you get two ADUs or one under BMC 12.14.135(C). New detached ADUs need full Washington State Energy Code compliance worksheets; King County or Snohomish Health District approval is required where the property is on septic and you are adding square footage or bedrooms |
| Pre-Approved Plans | None found in a City of Bothell source as of September 1, 2026 — the city's ADU submittal checklist assumes a custom set of construction plans and lists no pre-approved plan gallery. Confirm with Development Services at 425-806-6400 |
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Construction Costs [9]
Building an ADU in Bothell typically costs $235,000-$430,000 ($290-$480 per sq ft); a garage conversion runs $110,000-$235,000.
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| Average Build Cost | $235,000-$430,000 |
| Cost per Sq Ft | $290-$480 |
| Garage Conversion | $110,000-$235,000 |
| Prefab ADU (Installed) | $170,000-$330,000 |
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Rental Income Potential [10][11]
A 2-bedroom ADU in Bothell rents for around $2,250-$2,950/mo, an estimated 5-8% annual return.
| Avg Rent (1 Bedroom) | $1,800-$2,350/mo |
| Avg Rent (2 Bedroom) | $2,250-$2,950/mo |
| Short-Term Rental | Confirm with the City of Bothell at 425-806-6400 before you list. No short-term rental chapter of the Bothell Municipal Code could be verified for this entry — the code host blocks automated access — and BMC 12.14.135 itself imposes no minimum tenancy. Bothell's zoning does separately regulate "Rooms for roomers or boarders" as a permitted residential use in every residential zone (BMC 12.06.140(A)). Long-term letting of an ADU is unrestricted: there is no owner-occupancy rule and no cap on renting the unit |
| Est. Annual ROI | 5-8% annual return |
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Recent Law Changes
What's Changed Recently
- Washington HB 1337 (2023 c 334), codified at RCW 36.70A.681, preempts local ADU regulation in cities planning under the Growth Management Act: two ADUs on every lot in a zoning district inside an urban growth area that allows single-family homes, no owner-occupancy requirement, no maximum floor area under 1,000 sq ft, no roof height limit under 24 feet, no setbacks or yard coverage limits more restrictive than for the principal house, mandatory allowance of conversions of existing structures including detached garages, detached ADUs at a lot line abutting a public alley, impact fees capped at half the principal unit's, no public street improvements as a condition, and no off-street parking within half a mile walking distance of a major transit stop
- Bothell brought BMC 12.14.135 into line in stages — Ordinance 2415 § 5 (Exh. D) in 2024, Ordinance 2445 § 6 (Exh. F) in 2024 and Ordinance 2461 § 1 (Exh. A) in 2025 — raising the allowance to two ADUs per lot, exempting detached ADUs from the five-percent accessory building size cap, setting the detached height limit at 30 feet (33 over an existing accessory structure), adding an approval route for ADUs built without a permit, and codifying the 50 percent impact fee cap
- BMC 12.14.135(C) allows one accessory dwelling unit on lots designated with critical areas or their buffers. This is a genuine local grant, not a restatement of state law: RCW 36.70A.681(4) exempts those lots from every one of the statute's ADU protections, so in most Washington cities a critical-area designation leaves you with whatever the old local code said
- Bothell City Council approved the elimination of required parking minimums for new developments on July 8, 2025, citing SB 5184, alongside new Neighborhood Scale Commercial standards. Whether the repeal reached the one-space-per-ADU row in BMC 12.16.030 could not be verified in a city source for this entry
- Ordinance 2415 (2024) added BMC 12.16.110(F), waiving the ADU parking requirement not only within half a mile walking distance of a major or frequent transit stop, as the state requires, but also within half a mile walking distance of pedestrian access to a qualifying regional trail
- Bothell renamed its residential zoning classifications — the code's Residential Development Standards table now runs R-C, R-L1, R-L2, R-M1, R-M2, R-M3, R-M4 and R-AC, where older city documents and maps use R 40,000, R 9,600, R 8,400, R 7,200, R 5,400, R 4,000 and R 2,800
- The most recent version of the Bothell Municipal Code that could be retrieved for this entry is current through Ordinance 2464, passed September 16, 2025. The city has since published a Draft Housing Action Plan for virtual open houses, announced August 5, 2026 — confirm nothing has changed before you rely on any figure here
Tips for Building an ADU in Bothell
- You can build two, and on a critical-area lot you can still build one. BMC 12.14.135(C) is the provision to quote if a plan reviewer or a contractor tells you a wetland or steep-slope designation kills the project — state law would, Bothell's code does not
- The single most valuable line in Bothell's ADU section is the exemption from the five-percent accessory building rule. Without BMC 12.14.135(D)(2) a 1,200 sq ft detached unit would need a 24,000 sq ft lot; with it, hard surface coverage becomes the binding constraint instead
- Converting existing space beats building new on size: a conversion may exceed 1,200 sq ft with the community development director's approval, up to 40 percent of the house's gross floor area
- If your lot adjoins an alley, the rear setback drops from 15 feet (R-L1 and R-L2) or 35 feet (R-C) to zero — 3 feet for a garage door. That is the difference between a buildable backyard and an unbuildable one on a shallow lot
- Check the trail waiver before you plan parking. BMC 12.16.110(F) waives the ADU parking requirement within half a mile walking distance of pedestrian access to a qualifying regional trail as well as of a transit stop — and Council voted in July 2025 to scrap parking minimums for new development, so ask whether any requirement survives at all
- Get your address assigned first. The city will not accept the ADU application without the assigned address letter, and if Bothell is your water provider the water and sewer availability certificate takes up to two weeks on its own
- Already have an unpermitted unit? BMC 12.14.135(F) gives you a route: apply, take the building inspection — the ADU application fee covers it — and get it legal rather than waiting for a complaint
- Hold the city to the impact fee cap. It is in Bothell's own code, not just the statute: no more than 50 percent of what the principal unit would pay (BMC 12.14.135(G))
ADU Builders in Bothell
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Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- Bothell Municipal Code 12.14.135 — Accessory dwelling units (Ord. 2461 § 1 (Exh. A), 2025; Ord. 2445, 2024; Ord. 2415, 2024) Municipal Code — Accessed 2026-09-01
- Bothell Municipal Code 12.14.030 — Residential area and dimensions (Residential Development Standards table) Municipal Code — Accessed 2026-09-01
- Bothell Municipal Code 12.06.140 — Residential uses (use table and development conditions) Municipal Code — Accessed 2026-09-01
- Bothell Municipal Code 12.14.070 — Setbacks, applying to specific building or use Municipal Code — Accessed 2026-09-01
- Bothell Municipal Code 12.16.030 and 12.16.110 — Computation of required off-street parking; transit provisions Municipal Code — Accessed 2026-09-01
- RCW 36.70A.681 — Accessory dwelling units, limitations on local regulation (2023 c 334 s 4, HB 1337) state_statute — Accessed 2026-09-01
- Accessory Dwelling Unit submittal checklist (form 3f), City of Bothell Development Services, updated December 2025 — 2026 fees Planning Department — Accessed 2026-09-01
- Council Adopts City Code Changes for Parking and Small Business Permitting — City of Bothell news, July 9, 2025 Planning Department — Accessed 2026-09-01
- Angi — ADU Construction Cost Data Market Data — Accessed 2026-09-01
- Zillow Rental Manager — Market Rent Data Market Data — Accessed 2026-09-01
- HUD Fair Market Rents Government Data — Accessed 2026-09-01
- Washington State Office of Financial Management — April 1, 2026 Population of Cities, Towns and Counties Government Data — Accessed 2026-09-01
Frequently Asked Questions
Can I build an ADU in Bothell?
Yes. ADUs are allowed in Permitted (P) in every residential zone — R-C, R-L1, R-L2, R-M1, R-M2, R-M3, R-M4, R-MHP and R-AC — and not permitted in the OP, NB, CB, GC and LI zones (Bothell Municipal Code 12.06.140(A), Residential Uses table). Development condition 4 states that "Accessory dwelling units are permitted when added to, created within, or detached from a primary dwelling unit(s) subject to the restrictions in BMC 12.14.135" (BMC 12.06.140(B)(4)). Bothell renamed its residential zones as part of its 2024-2025 code updates; older documents still use the R 40,000 / R 9,600 / R 8,400 / R 7,200 / R 5,400 / R 4,000 / R 2,800 labels. RCW 36.70A.681(1)(c) independently requires two ADUs in all zoning districts inside an urban growth area that allow single-family homes. Confirm your parcel's zone with Bothell Community Development at 425-806-6400 in Bothell. The maximum size for a detached ADU is 1,200 sq ft. "An accessory dwelling unit may be developed up to 1,200 square feet" (Bothell Municipal Code 12.14.135(D)(1)) — 200 sq ft above the 1,000 sq ft floor state law guarantees (RCW 36.70A.681(1)(f)). Conversion of an existing space may exceed 1,200 sq ft as approved by the community development director, provided it does not exceed 40 percent of the primary dwelling unit's gross floor area. Critically, "Detached accessory dwelling units are exempt from the provisions of BMC 12.14.130 that limit the size of accessory buildings to five percent of the lot area" (BMC 12.14.135(D)(2)) — without that exemption a 1,200 sq ft detached unit would need a 24,000 sq ft lot. Owner-occupancy is not required, so you can rent out the ADU.
How much does an ADU cost in Bothell?
ADU construction in Bothell typically costs $235,000-$430,000. Garage conversions run $110,000-$235,000, and prefab ADUs cost $170,000-$330,000.
How much can I rent an ADU for in Bothell?
Average ADU rents in Bothell are $1,800-$2,350/mo for a 1-bedroom and $2,250-$2,950/mo for a 2-bedroom unit.
Do I need parking for an ADU in Bothell?
One off-street space per ADU as last verifiable, waived on transit or trail proximity — and possibly now zero. BMC 12.16.030(A) lists "Dwelling units, accessory — 1 per unit, unless waived per BMC 12.16.110(F)", and BMC 12.16.110(F) states the ADU parking requirement does not apply where the ADU is within one-half mile walking distance of a major or frequent transit stop, or within one-half mile walking distance of pedestrian access to a regional trail that provides a continuous two-way paved connection to employment centers or similar activity centers and amenities (Ord. 2415, 2024) — the trail waiver is Bothell's own addition and reaches much of the Sammamish River and Burke-Gilman corridor. Since then, on July 8, 2025, the City Council "approved the elimination of required parking minimums for new developments," citing SB 5184. Whether that repeal reached the single-family ADU row could not be verified for this entry because Bothell's code host blocks automated access — confirm with Bothell Community Development at 425-806-6400 before you give up a parking space. Either way, RCW 36.70A.681(2)(a) caps what Bothell may require at zero within half a mile walking distance of a major transit stop, one space per unit on lots under 6,000 sq ft, and two per unit on larger lots. Check with Bothell planning department for the latest Washington ADU parking rules, as many states have relaxed requirements in recent years.
How long does it take to get an ADU permit in Bothell?
Bothell does not publish a standard ADU review timeline — confirm with Bothell Development Services at 425-806-6400 — no ADU review timeline is published on the city's ADU submittal checklist. Budget separately for the water and sewer availability certificate, which the checklist puts at up to two weeks where the City of Bothell is the provider. Permit fees: At submittal (2026 fees, from the city's ADU checklist): a plan check fee based on project valuation, a Single Family Planning Review fee of $231.00, and a 5 percent technology fee. At permit issuance: the permit fee, plumbing, mechanical and state fees; a Residential Site Review and Inspection fee of $834.00 where you are constructing an addition or a new building; a stormwater facility charge if you are adding square footage; and the 5 percent technology fee on everything except the state fee, impact fees and facility charges. For a new detached ADU the plumbing and mechanical permit fees are each 8 percent of the building permit fee, 16 percent in total; for a remodel or addition creating an ADU they are a base fee of $82.00 plus a per-fixture fee, most residential fixtures running $27.00 each.
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