What happens if you skip the permit (and you needed one)
- Stop-work orders and fines: Bainbridge Island Building Department will issue a cease-work notice (typically $300–$750 per day of continued work after notice) and require a retroactive permit application, which costs 50–100% more than the original permit fee.
- Insurance and lender denial: Your homeowner's insurance will not cover an unpermitted ADU structure; if discovered during a claim, your policy may be voided. Lenders and title companies now flag unpermitted structures during refinance or sale — expect 10–30% reduction in final sale price or outright deal rejection.
- Property transfer disclosure: Washington requires all unpermitted structures be disclosed on the Property Condition Disclosure Form (WA RCW 64.06); buyers can rescind or sue for damages. On Bainbridge Island, this is a fatal flaw — the market will not absorb undisclosed unpermitted construction.
- Demolition order and lien: If the ADU violates shoreline setbacks or Design Review standards, the City can order removal; costs $15,000–$40,000 for deconstruction, and a lien will attach to your property until paid.
Bainbridge Island ADU permits — the key details
Washington State Law RCW 36.70A.696 (effective January 1, 2024) mandates that cities allow at least one ADU per single-family residential lot, and prohibits local governments from imposing minimum lot-size requirements, owner-occupancy mandates, or parking minimums on ADUs. However, Bainbridge Island's 2023 ADU ordinance (Chapter 16.24 BMC) adds local conditions: the ADU must comply with Shoreline Master Program setbacks (if within SMP jurisdiction, typically 50–100 feet from ordinary high water mark for water-adjacent properties), cannot exceed 1,200 square feet for a detached ADU or 800 square feet for a junior ADU (where junior ADU means interior space carved from the primary home), and must pass Design Review if visible from the water or within a historic district. Detached ADUs must maintain 15-foot side setbacks and 25-foot rear setbacks on standard residential lots (unless the lot abuts steep slopes, where setbacks increase). The city also requires a preliminary Shoreline Master Program assessment for any lot within the SMP jurisdiction (much of Bainbridge Island's east and west shores qualify), which adds 3–4 weeks to the review timeline and may require costly engineering reports if the ADU is near water, slope, or sensitive habitat.
The Bainbridge Island Building Department processes ADU permits through a two-track system: ministerial review (8–10 weeks for administrative approvals and plan review) plus discretionary review (Design Review, if required). Plan submissions must include a full building set (architectural, structural, electrical, plumbing, mechanical, and grading), a Shoreline Master Program environmental checklist or full assessment, proof of utility capacity (water, sewer, power), and a wetland/tree survey if the property is within sensitive-area buffers. The City's online permit portal (accessible via the City of Bainbridge Island website) allows document uploads, but most applicants work with a local permit expediter or architect due to the SMP complexity. Initial application fees are $1,200–$1,800 (plan-review deposit); building permit issuance is $2,200–$4,500 depending on valuation (typically 0.5–1% of construction cost); and Design Review (if required) is $800–$1,500. Total soft costs (survey, engineered grading, SMP assessment) run $3,000–$8,000 for water-adjacent or slope-sensitive lots. Most applicants budget $8,000–$12,000 in permits and engineering combined.
Egress, kitchen, and utility standards for Bainbridge Island ADUs follow the International Building Code (the City has adopted the 2021 IBC with Washington amendments). A detached ADU or garage-conversion ADU must have at least one egress window (IRC R310.1) serving a bedroom or living area, with a minimum clear opening of 5.7 square feet and a sill height no more than 44 inches above the floor — a critical requirement if you are converting a garage or building a small detached unit. A full kitchen (with sink, cooking appliance, refrigerator, and counter space) triggers full building code compliance (mechanical ventilation, electrical load calculations, plumbing venting); a kitchenette or wet bar does NOT. Junior ADUs (interior ADUs within the primary home, typically created by adding a separate entrance and kitchenette to an existing room or basement) are exempt from some mechanical-system duplication but must still have independent egress and meet energy-code standards. Separate utility connections are not explicitly required by Bainbridge Island ordinance, but the Water Department and Public Utility District often require a separate meter for billing and code compliance; if the ADU shares utilities with the primary home, the applicant must demonstrate adequate service capacity and obtain written utility approval. Most ADUs on Bainbridge Island get separate meters ($1,500–$3,000 for water, sewer, and electrical subpanels).
Owner-builder status: Bainbridge Island allows owner-builders to obtain a building permit for a primary residence or ADU on owner-occupied property. Owner-builders must file an Affidavit of Owner-Builder with the permit application (no contractor's license required). However, the City does NOT allow owner-builders to pull electrical, plumbing, or mechanical permits without a licensed contractor; you can frame, roof, and do finish carpentry, but you must hire licensed subs for the 'big four' trades. This hybrid approach means most owner-builders will still spend $15,000–$30,000 on licensed labor for the technical trades, even if they do the structural and finishing work. Inspections are mandatory at foundation, framing, rough-in (mechanical, electrical, plumbing), insulation/drywall, and final. The City schedules inspections through the permit portal; turnaround is typically 3–5 business days after you call for inspection.
Climate and soil considerations: Bainbridge Island's glacial-till soils on the western part of the island are well-drained but shallow-bedrock in places, requiring geotechnical assessment if the ADU site is on a slope. The eastern side of the island (toward the ferry) has deeper alluvial soils but higher groundwater, especially in winter. Frost depth is 12 inches for most residential areas, but the International Building Code (adopted by Washington and thus Bainbridge Island) now recommends frost depth of 36–48 inches below grade for new foundations in cold-climate zones due to freeze-thaw expansion — check with the City's structural reviewer to confirm local expectations. Roof snow load is 25 pounds per square foot (per ASCE 7 for this region), which affects roof framing. Seismic design category is D1, so detached ADU foundations must be rated for lateral loads. If your ADU site is on a slope steeper than 15 percent or within a critical root zone of a tree over 12 inches diameter at breast height, the City will require an arborist report and grading engineer plan, adding 2–3 weeks and $2,000–$5,000 to the application.
Three Bainbridge Island accessory dwelling unit (adu) scenarios
Shoreline Master Program (SMP) and water-adjacent ADU complexity
Bainbridge Island's Shoreline Master Program (Chapter 16.32 BMC) applies to all property within 200 feet of the ordinary high water mark (OHWM) of Puget Sound, Eagle Harbor, and Blakely Harbor. If your lot is within this zone — and roughly 40 percent of Bainbridge Island's residential parcels are — the ADU application triggers a mandatory SMP consistency review by the Planning Department. The SMP establishes setback requirements (typically 50–100 feet from OHWM, depending on the water body and shoreline type), limits on lot coverage, and environmental-impact assessments. For an ADU, this means the Planning Department will require an SMP environmental checklist or, in sensitive areas (critical habitat, eelgrass beds, riparian corridors), a full biological assessment. If your ADU site is within a sensitive-area buffer (typically 200–400 feet landward from OHWM), the City will require a qualified biologist's report addressing impacts to wetlands, streams, or wildlife habitat. This adds 3–4 weeks to the timeline and $2,000–$6,000 in consulting costs.
Practical implication: if your water-adjacent lot slopes down toward the water, or if a stream or wetland exists on the downslope side, the SMP will push the ADU location further upslope, which may conflict with hillside setbacks or tree-protection zones. You must work with a local environmental consultant or permit expediter to triangulate the SMP setback, the hillside-grading slope, and the tree-root zone before you finalize your design. Many water-adjacent ADU applications are rejected or require substantial redesign because applicants do not account for SMP overlap with other constraints. Budget 3–4 additional weeks and $4,000–$8,000 for SMP assessment and any required environmental study before you commit to a builder or major design investment.
The City posts its SMP consistency determination (whether the ADU is consistent or requires further study) before the building permit can issue. If the determination is conditional or requires mitigation (e.g., native-plant buffer, enhanced stormwater infiltration, or stream restoration), the City will impose conditions on the building permit and require proof of compliance at final inspection. Most water-adjacent ADUs now include a rain garden or bioswale in the design to manage stormwater runoff, which adds $3,000–$8,000 to construction cost but secures SMP approval.
Design Review and the Bainbridge Island Design Guidelines
Bainbridge Island's Design Review ordinance (Chapter 16.28 BMC) requires discretionary Design Review approval for certain ADUs, including any detached or visible structure within a historic district, or any structure visible from a water body or public right-of-way that may substantially alter the character of the neighborhood. Bainbridge Island's Design Guidelines (published by the Planning Department) emphasize context-sensitive design, native vegetation, and visual compatibility with the existing streetscape and natural environment. In practice, the City's Design Review Officer or Hearing Examiner will evaluate the ADU's exterior color, materials, roof pitch, window patterns, mechanical equipment screening, and landscape buffering against these guidelines. An ADU that mimics the architectural style of the primary home, uses natural materials (cedar siding, asphalt or metal roofing), and is well-screened by existing vegetation or new native plants will sail through Design Review in 4–6 weeks. An ADU with metal siding, a flat roof, bright white exterior, or prominent HVAC units on the roof will trigger a Hearing Examiner review and may be denied or conditioned on substantial redesign.
Applicants often underestimate the cost and timeline of Design Review. A Design Review package requires architectural elevations (exterior views from all four sides), a site perspective (showing the ADU in context with the existing home and neighborhood), a tree survey, and a landscape plan with native plant species identified and sized at maturity. Many applicants hire an architect or designer to prepare these documents ($1,500–$3,000), then a landscape architect to refine the plantings ($1,000–$2,000). The City's Planning Department may also require a peer-review by a design consultant (paid by the applicant) if the application is contentious; this can add another $1,500–$3,000 and 2–3 weeks.
Timing: Design Review happens in parallel with or before the building-permit plan review. If you submit the ADU application with the Design Review package, the Planning Department will schedule the Design Review hearing (or administrative review if the application is deemed ministerial) within 3–4 weeks. Approval or conditional approval typically comes within 6–8 weeks. Then you revise the architectural and landscape plans per the Design Review decision and resubmit for building-permit plan review. Most applicants experience a 4–6 week delay between initial ADU application submission and building-permit issuance due to Design Review.
281 Winslow Way E, Bainbridge Island, WA 98110
Phone: (206) 842-2700 | https://www.ci.bainbridge-isl.wa.us/permits (or contact City to confirm current portal URL)
Monday–Friday, 8:00 AM–5:00 PM (verify hours on City website before visit)
Common questions
Does Bainbridge Island require owner-occupancy of the primary home to build an ADU?
No. Washington State Law RCW 36.70A.696 (effective 2024) prohibits cities from requiring owner-occupancy of the primary home. Bainbridge Island's 2023 ADU ordinance does not impose an owner-occupancy requirement. You may build an ADU and rent it out without living in the primary home, subject to all other local requirements (SMP, Design Review, setbacks, etc.). The rental income is yours; there is no local rent-control or affordability mandate for ADUs on Bainbridge Island (unlike some California cities). Homeowners Association (HOA) rules may restrict rentals — check your CC&Rs before you invest in the ADU.
What is the maximum size of an ADU on Bainbridge Island?
Bainbridge Island limits detached ADUs to 1,200 square feet and junior ADUs (interior ADUs within the primary home) to 800 square feet. Above-garage ADUs are treated as detached ADUs and subject to the 1,200 sq ft limit. These limits are set by Bainbridge Island's 2023 ordinance (Chapter 16.24 BMC) and are more restrictive than Washington State's statutory floor (state law does not cap ADU size, only lot-size requirements). If you need a larger ADU, you must apply for a variance, which is discretionary and rarely granted. Budget on the 1,200 sq ft cap for a detached ADU.
Can I have more than one ADU on my residential lot?
No. Bainbridge Island allows only one ADU per single-family residential lot, per state law and local ordinance. You cannot stack multiple ADUs (e.g., a detached ADU plus a junior ADU) or add an ADU to a lot that already has an accessory dwelling. State law allows cities to permit multiple ADUs on larger (multifamily-zoned) lots, but Bainbridge Island has not adopted this provision. Stick to one ADU per parcel.
Does Bainbridge Island require a separate water and sewer meter for the ADU?
Bainbridge Island's ordinance does not explicitly mandate separate meters, but the Water Department and Public Utility District typically require one for billing and code compliance. If you share utilities with the primary home, you must obtain written utility approval and demonstrate adequate capacity. Most applicants choose separate meters to avoid future disputes and simplify utility management; separate meters cost $1,500–$3,000 for water, sewer, and electrical connections. Check with the Bainbridge Island Water Department and PUD early in the planning process to confirm their meter requirements.
How long does a building permit take in Bainbridge Island for an ADU?
For a straightforward detached ADU in an upland (non-SMP) location with no Design Review required, permit approval typically takes 8–10 weeks from initial application to issuance. If the ADU is water-adjacent (SMP applies) or requires Design Review, add 4–8 weeks for the SMP assessment and Design Review process. If the lot is hillside or has tree-protection complications, add another 2–4 weeks for arborist and geotechnical review. Total timeline from application to building-permit issuance: 8–18 weeks depending on site complexity. After permit issuance, construction inspections and closeout take 6–12 weeks depending on your pace and the inspector's availability. Plan for 4–6 months from application to final occupancy.
Is architectural design required for an ADU application?
Yes. Bainbridge Island requires a full building set for all ADU applications, including architectural plans (floor plans, sections, elevations), structural calculations, electrical one-line diagram, plumbing and mechanical layouts, and grading/site plan. You cannot proceed with a sketch or verbal description. Many applicants hire an architect or designer ($3,000–$8,000) to prepare these documents. Some local architects specialize in ADU design and can deliver a permitted set in 4–6 weeks. If you use a pre-designed ADU plan from a third party (common in California but less common in Washington), you will still need to tailor it to Bainbridge Island's specific setbacks, SMP requirements, and local code amendments — expect 1–2 weeks of revision work and $1,000–$2,500 in local design adjustments.
Can I build an ADU as an owner-builder without a general contractor's license?
Yes, with limitations. Bainbridge Island allows owner-builders to pull a building permit for a primary residence or ADU on owner-occupied property. You must file an Affidavit of Owner-Builder with your permit application. However, you cannot pull electrical, plumbing, or mechanical permits without a state-licensed contractor. You can do framing, roofing, finish carpentry, and site work yourself, but you must hire licensed electricians, plumbers, and HVAC contractors for their trades. Most owner-builders hire a GC to manage scheduling and inspections, which costs 15–25% of the construction budget. If you go fully DIY (owner-builder for construction trades too), you will still need licensed subs for the permit-pulling aspects, which saves money but adds coordination complexity.
What happens at the final inspection for an ADU?
The final inspection includes verification that the ADU is built to the approved plans, all mechanical systems (HVAC, plumbing, electrical) are code-compliant and functional, egress windows are proper size and operable, kitchen and bathroom fixtures are installed, and the ADU is safe for occupancy. The City's Building Inspector will walk through with you or your contractor and sign off on code items. For water-adjacent ADUs, the Planning Department will also sign off to confirm SMP conditions (e.g., native plants, stormwater infiltration) are met. Once the Building Inspector approves, you receive a Certificate of Occupancy and can legally occupy or rent the ADU. Expect 1–2 weeks for the City to schedule the final inspection after you request it.
Are there any tax or property-assessment implications of adding an ADU?
Yes. Adding an ADU will increase your property's assessed value, which typically increases your property tax bill. The amount depends on the local assessed value and tax rate. On Bainbridge Island, a $200,000 ADU construction cost typically results in a $15,000–$25,000 increase in assessed value and a $300–$500 annual increase in property taxes (at ~2% local tax rate). Check with the Island County Assessor's office before you build to estimate the tax impact. Some states (e.g., California) offer property-tax exemptions for ADUs; Washington does not currently offer a state-level ADU property-tax exemption, though Bainbridge Island has discussed local incentives. ADU rental income is taxable as regular income on your federal return; consult a tax professional on deductions and depreciation for rental property.
What is the difference between a junior ADU and a detached ADU on Bainbridge Island?
A junior ADU is carved from the interior space of the primary home (e.g., converting a basement or garage interior into a one-bed/one-bath ADU with separate entrance and kitchenette). A detached ADU is a separate building on the lot (e.g., a new cottage, above-garage unit, or converted garage structure). Bainbridge Island caps junior ADUs at 800 sq ft and detached ADUs at 1,200 sq ft. Junior ADUs are generally faster to permit (7–9 weeks, less site complexity) and cheaper to build because they reuse the primary home's foundation and utilities (if shared). Detached ADUs take longer (8–12 weeks, more site review), cost more to build (new foundation, utilities, grading), but give you more flexibility on lot location and allow you to eventually separate the unit as a true duplex if zoning changes. Both require a full building permit and the same plan review process; the main difference is construction scope and timeline.