BEBuild Eastside

Bothell ADU and detached ADU rules

Bothell ADU and DADU rules under the current 2025–2026 code

Bothell's adopted ADU code is substantially more flexible than several older application packets still found online. It generally allows two ADUs, removes owner-occupancy and special entrance rules, and does not contain an ADU-specific parking minimum. It limits a middle-housing or critical-area/buffer lot to one ADU.

Reviewed August 14, 2026 against adopted city code and official city guidance

Quick definitions. An accessory dwelling unit (ADU) is a complete additional home on the same property. A detached ADU (DADU) is in its own building. FAR or GFA limits how much enclosed floor area the property may contain.

Bottom line

What the current rules mean

A typical eligible residential lot may have two ADUs, each up to 1,200 square feet. A DADU may reach 30 feet, or 33 feet above an existing accessory structure, and uses the primary building's zone-based rear yard. The exact answer depends on whether the property contains a critical area or buffer, its zoning and hard-surface limit, utility provider, fire flow, trees, drainage, and whether an older structure or unapproved ADU is being converted.

2 / 1ADU count by siteTwo generally; one with middle housing or a designated critical area/buffer.
1,200 sq ftTypical maximumA limited existing-space conversion may be larger.
30 / 33 ftDetached heightThe higher cap applies above an existing accessory structure.

What is allowed on one lot

Bothell generally allows two ADUs on a residential lot. Only one ADU is allowed when the lot is developed with middle housing, and only one is allowed when the lot is designated with a critical area or buffer. The use table allows ADUs in R-C, R-L1, R-L2, RM-1 through RM-4, R-MHP and R-AC, subject to each zone and site condition.

  • Two attached or internal ADUs on a typical eligible lot.
  • One attached and one detached ADU.
  • Two detached ADUs where zoning, building/fire separation, hard surface and access allow them.
  • Only one ADU in any configuration on a middle-housing site or a lot designated with a critical area or buffer.

Official sources: BMC 12.14.135: accessory dwelling units · BMC 12.06.140: residential allowed-use table · RCW 36.70A.681: limits on local ADU regulation

Size, height, setbacks, and site capacity

Maximum size
The maximum is 1,200 square feet. The Director may approve conversion of existing space above 1,200 square feet, but not above 40 percent of the principal dwelling's gross floor area. DADUs are exempt from the general accessory-building cap of five percent of lot area.
Height
A DADU may be up to 30 feet, or 33 feet when located above an existing accessory structure. An attached ADU uses the principal building standards.
Setbacks
A DADU uses the primary building's rear setback. Typical fronts are 15 feet in R-C, R-L1 and R-L2 and 10 feet in RM zones; typical sides are five feet. Rear yards are generally 35 feet in R-C, 15 feet in R-L1/R-L2 and 10 feet in RM zones. A qualifying alley line may use zero feet, with three feet where a garage door faces the alley. Corner, through-lot, easement, utility and critical-area rules can change these numbers.
FAR, lot coverage, or impervious area
The DADU's exemption from the five-percent accessory-building cap does not exempt it from the zone's overall hard-surface limit. Current limits include 35 percent in R-C, 55 percent in R-L1, 60 percent in R-L2, 70 percent in RM-1, and generally 75 to 90 percent in denser RM zones. No general FAR applies across these zones; hard surface, impervious area, landscaping and stormwater are the important calculations.

Official sources: BMC 12.14.135: accessory dwelling units · BMC 12.14.030: residential dimensions · BMC 12.14.070: rear yards

Parking, occupancy, renting, and separate sale

Parking
The enacted ADU section contains no ADU-specific parking minimum. An earlier 2025 draft proposed one stall and a transit exception, but that language was not adopted. The current checklist asks the site plan to show parking. Confirm the general parking and site-review treatment rather than assuming either a required stall or a universal waiver.
Owner occupancy
The current ADU section contains no owner-occupancy requirement.
Short-term rental
The current ADU section contains no rental-duration or ADU-specific short-term-rental restriction. General business, rental, nuisance and private-covenant requirements still apply and should be confirmed for the intended use.
Separate sale
The current ADU section does not impose a separate-sale prohibition, but separate condominium or fee-simple ownership still requires a lawful condominium, subdivision or other title route.

Official sources: BMC 12.14.135: accessory dwelling units · RCW 36.70A.681: limits on local ADU regulation

Utilities, fire access, and the permit path

Apply through MyBuildingPermit using Bothell's single-family residential ADU workflow and obtain an assigned address. Water-meter sizing, water/sewer availability, side sewer or septic approval, fire flow, drainage and utility easements depend on the serving district. Bothell spans King and Snohomish Counties and parcels may be served by Bothell, Alderwood, Northshore, Woodinville or another utility provider.

  1. Confirm that the parcel is inside Bothell, identify its county, zone, utility providers, sewer/septic system, and whether any critical area or buffer is designated.
  2. Calculate primary-building setbacks, hard surface, height, trees, drainage, easements, access and fire flow; do not apply the ordinary five-percent accessory-building cap to a DADU.
  3. For an existing unapproved ADU, submit the required legalization application and plan for City inspection and corrective permits.
  4. Apply through MyBuildingPermit with Bothell's ADU checklist, address, site plan and all applicable water-meter, utility-availability, fire-flow, septic, storm, tree and structural documents.
  5. Use the current fee and impact-fee schedules and confirm school-district treatment rather than carrying forward an older packet total.

Official sources: Bothell current ADU permit checklist · Bothell permit applications and forms · Bothell 2026 impact-fee schedule

Rules that can change the answer for one property

A citywide ADU allowance does not prove that a detached unit fits on a particular lot. Resolve these items before buying plans or fixing the DADU location.

  • A designated critical area or buffer reduces the maximum from two ADUs to one even when the proposed footprint lies elsewhere.
  • The primary-building rear setback applies to a DADU; older accessory-building rear-yard assumptions can produce a false fit.
  • The correct utility and health agencies depend on the parcel's service area and whether it lies in King or Snohomish County.
  • Hard-surface, stormwater, significant-tree and off-site dripline rules can be more limiting than the nominal yards.
  • Fire flow, access and adopted building/fire-code configuration decide whether additional protection or sprinklers are needed; ADU status alone does not answer it.

Official sources: Bothell Permit Center · BMC 12.18.030: tree retention · Bothell tree-code update project

Prepared for a future address check

What a reliable DADU feasibility screen must verify

Bothell's DADU screen should begin with city/county, zone and the parcel-wide critical-area/buffer flag, then apply primary-building yards, 30- or 33-foot height, hard surface, trees, drainage, utilities, fire flow and legal-structure history.

Jurisdiction and zoning

Confirm the address is actually governed by Bothell and select the correct base-zone standards.

Existing and proposed units

Count the principal dwelling, existing legal ADUs, middle-housing units, and the proposed attached or detached configuration.

Surveyed parcel geometry

Use legal lot lines, front/rear designation, alleys, access easements, and existing structures rather than a tax-map rectangle.

Capacity calculations

Test the ADU against the rules that apply locally: FAR or GFA, building or lot coverage, impervious or hard surface, and landscaping.

Mapped and field constraints

Screen critical areas and buffers, shorelines, slopes, flood hazards, significant trees, drainage, and recorded easements.

Utilities and emergency access

Verify water, sewer or septic, fire flow, hydrant and apparatus access, power, meters, and trench routes.

The engine should not import the one-stall, owner-occupancy, entrance, or rental restrictions from older packets. It also should not promise a parking waiver or sprinkler result without current general-code and site review.

Do not rely on the older rule

Earlier Bothell packets and drafts can show one ADU, one parking stall, owner occupancy, entrance placement, or rental restrictions. Those provisions do not appear in current BMC 12.14.135. Use the enacted current code and the checklist updated in December 2025.

Primary sources

Official Bothell and Washington references

These are the sources used for this guide. Check them again before filing because ordinances, fee schedules, and permit forms can change.

BMC 12.14.135: accessory dwelling unitsCurrent count, critical-area/middle-housing limits, size, height, legalization, and impact-fee cap.BMC 12.06.140: residential allowed-use tableCurrent residential zones in which ADUs are allowed.BMC 12.14.030: residential dimensionsCurrent zone setbacks, hard-surface limits and alley conditions.BMC 12.14.070: rear yardsCurrent rule applying the primary-building rear yard to a DADU.Bothell current ADU permit checklistMyBuildingPermit, submittal, address, utility, fire-flow, drainage, critical-area and fee guidance updated December 2025.Bothell permit applications and formsCurrent application, utility, building, fire, site and supporting forms.Bothell 2026 impact-fee scheduleCurrent park, transportation and fire impact-fee treatment for ADUs.Bothell Permit CenterCurrent critical-area, land-use, building, tree and permit resources.BMC 12.18.030: tree retentionCurrent significant-tree, retention, protection and replacement standards.Bothell tree-code update projectOfficial update status; the pending project does not replace current enacted tree code.RCW 36.70A.681: limits on local ADU regulationWashington's current baseline for two ADUs, size, height, parking, owner occupancy, conversions, alleys, fees, and critical-area exceptions.

A practical first-pass checklist

  1. 1

    Confirm the parcel is inside Bothell city limits and identify its current zone.

  2. 2

    Map the principal home, lot lines, easements, trees, utilities, driveway, critical areas, and buffers on one current survey.

  3. 3

    Test the ADU footprint against setbacks, height, floor-area or coverage limits, hardscape, parking, and construction access.

  4. 4

    Ask the utility providers to confirm water, sewer, power, meter, side-sewer, and capacity requirements before fixing the design.

  5. 5

    Have the city confirm the complete land-use, building, fire, right-of-way, tree, and utility permit list in writing.