Kitsap County ADU Rules: Size, Setbacks, Water, and Permits

Kitsap County ADU rules split along one line: whether your property sits inside an urban growth area (UGA) or outside one. Urban lots get the more permissive treatment thanks to Washington’s 2023 ADU law, RCW 36.70A.681, including no owner-occupancy requirement and up to two ADUs per lot. Rural lots are capped at one ADU, with owner-occupancy still required for detached units and a conditional use permit on top of the building permit. The current standards live in Chapter 17.415 of the county code, not the older Section 17.410.060 that some out-of-date guides still cite.¹

If you’re not sure which category your lot falls into, the Kitsap County Department of Community Development can tell you, and zoning maps are available through their permit portal.

Urban Growth Area vs. Rural: Why It Matters

Inside a UGA, you can build attached and detached ADUs, and state law prohibits the county from requiring you to live on the property.² You can rent both the main house and the ADU. Urban ADUs follow standard zoning setbacks, height limits, and lot coverage for the underlying zone, and any ADU built entirely inside an existing structure is exempt from current setback and lot coverage standards as long as you don’t expand the footprint.¹

Outside a UGA, everything tightens. Rural lots are limited to one ADU. For detached rural ADUs, the owner must live in either the main home or the ADU. A rural attached ADU needs only a building permit, but a rural detached ADU requires a conditional use permit as well, which adds both time and cost.¹

How Many ADUs You Can Build

State law requires Kitsap County to allow at least two ADUs on lots within a UGA that permit single-family homes. The allowed combinations are one attached and one detached, two attached, or two detached (which may share a single structure or occupy two separate buildings).² On very small lots of 2,000 square feet or less, the county may cap the total at two ADUs in addition to the primary home.

Rural lots are limited to one ADU. An attached ADU is not permitted where a detached one already exists, and vice versa. The state’s two-ADU mandate does not extend to rural zones.¹

There’s a significant carve-out. The state’s two-ADU allowance does not apply to lots designated with critical areas or their buffers, or to watersheds serving potable-water reservoirs listed as impaired under the federal Clean Water Act.² Given Kitsap’s extensive shoreline and wetland acreage, this exception affects more properties here than in most Washington counties.

Size Limits

Size rules depend on the zone and the type of ADU. All dimensions use interior measurements.

Urban growth area: an ADU cannot exceed 1,000 square feet. The county director may allow equal square footage for the primary dwelling and the ADU if the ADU occupies a single floor of the existing home.¹
Rural attached ADU: limited to 50 percent of the habitable area of the primary residence.
Rural detached ADU: cannot exceed 50 percent of the habitable area of the primary residence or 900 square feet, whichever is smaller. The unit must sit within 150 feet of the primary residence unless it’s a conversion of an existing structure such as a garage.

Mobile homes and recreational vehicles cannot serve as ADUs in rural areas.

Setbacks, Height, and Critical-Area Buffers

Kitsap County doesn’t set ADU-specific setbacks. Instead, ADUs follow the standard setback, height, and lot coverage rules for whichever zone the property sits in, so you’ll need to look up your zoning designation for the exact numbers.¹ Two urban exceptions help: a detached ADU doesn’t need to meet setback requirements along a public alley, and an ADU built entirely inside an existing structure is exempt from current setbacks and lot coverage standards as long as the footprint doesn’t grow.

For rural detached ADUs, the code requires the unit to maintain the appearance of the primary residence. That doesn’t mean identical, but the county expects visual compatibility with the main home. Urban ADUs don’t carry a similar design-matching requirement.

Critical-area buffers stack on top of standard setbacks, and the largest applicable buffer controls.³ Shoreline setbacks range from 50 feet in high-intensity zones up to 200 feet in natural designations, with an additional 15-foot building setback beyond the buffer in every case. Wetland buffers vary by category: Category IV wetlands require 40 feet, Category III wetlands require 110 feet, each plus a 15-foot building setback. Any development within 250 feet of a known or potential wetland requires a wetland report. Streams have their own buffers: 150 feet for fish-bearing streams and 50 feet for non-fish-bearing streams, plus a 15-foot building setback.

Steep slopes add another layer. Moderate hazard slopes (15 to 29 percent grade) require a 40-foot setback from the top or toe. High hazard slopes (30 percent or steeper) require a setback equal to the slope height plus one-third of that height, or 25 feet, whichever is greater. A geotechnical report is required for any development within 200 feet of a slope over 30 percent.

Check critical-area status early. If your lot falls within a critical area or its buffer, the state’s two-ADU allowance may not apply at all.²

Parking

State law limits how much parking counties can require for ADUs, and the rules vary by lot size and proximity to transit:²

Within half a mile of a major transit stop: no off-street parking can be required.
Lots under 6,000 square feet: no more than one parking space per ADU.
Lots 6,000 square feet or larger: no more than two parking spaces per ADU.

The rural code still requires ADUs to provide additional off-street parking and to use the same side-street entrance as the primary residence. Rural attached ADUs are specifically prohibited from adding a separate street-side entrance.¹

Water and Septic Approval

Every ADU must meet health district standards for water and sewage.¹ Public sewer is straightforward. Septic is where projects most often stall.

You’ll need a Residential Building Clearance from the Kitsap Public Health District. To qualify, your property must have a permitted septic system with a designated reserve area. If your system was never formally permitted, you’ll go through a separate, more involved process before the Health District will approve the project.⁴ Properties on private wells must demonstrate full water adequacy, and ADUs are specifically listed as a project type requiring that proof.

If the ADU connects to the existing septic system, you also need a separate Connection Permit. The Health District inspects the physical connection before it’s covered, and your building permit final can’t be issued until that connection inspection passes. The Health District’s review runs on its own timeline, so submit early.

Fire Sprinklers

The Kitsap County Fire Marshal’s Office handles fire safety on a case-by-case basis. If the property lacks adequate fire access (generally a 20-foot-wide road under the International Fire Code), the Fire Marshal may require the ADU to have a fire sprinkler system. Properties on narrow private roads or long driveways are the most likely to trigger this. It isn’t automatic, but when it applies it adds meaningful cost.

Permits, Fees, and Pre-Approved Plans

Applications go through the Department of Community Development’s online Permit Application Portal. You’ll need a scaled site plan showing existing and proposed structures, property lines, setbacks, and utility locations, plus architectural drawings including floor plans and elevations.

Building permit fees are calculated by multiplying the project’s square footage by a per-square-foot construction cost (published annually by the International Code Council), then multiplying that total valuation by a permit fee multiplier of 0.0214 for residential projects. That multiplier covers plan review, and for new single-family residential construction it also folds in mechanical and plumbing permits.⁵ A $90 base application fee and a 3 percent technology surcharge apply to every permit.

Rural detached ADUs require a conditional use permit. An administrative CUP costs $3,045. A standard CUP costs $3,915 plus a $650 hearing examiner fee, totaling $4,565.

Impact fees for roads, schools, and parks are collected separately. State law caps ADU impact fees at 50 percent of what would be charged for a principal dwelling.² Under Kitsap’s current fee table, road impact fees run about $2,529 for a detached ADU and about $1,534 for an attached ADU. School and park impact fees are added on and vary by location.⁶ Properties using a new permit-exempt well pay an additional $500 water withdrawal fee.

Kitsap offers a set of pre-approved ADU plans that can speed permitting considerably. The plans come in several sizes with published permit-fee estimates:⁷

Lille, 480 sq ft: about $4,667
Sidney, 600 sq ft: about $5,120
Sinclair, 800 sq ft: about $5,847
Blakely, 800 sq ft (two-story): about $5,770
Kitsap, garage conversion (two- or three-car): about $7,235
Salish: about $6,594

The plans are free to use in urban unincorporated Kitsap County and within the cities of Bremerton, Bainbridge Island, Port Orchard, and Poulsbo, provided you don’t modify the structural plans. In rural areas, a $1,000 design fee applies on top of the CUP fees. Non-structural customization is allowed: window colors, siding materials, roof pitch and style, door choices, and non-load-bearing interior walls. Using a pre-approved plan doesn’t eliminate the permit process, but it removes the structural plan-review back-and-forth that adds weeks to a custom design.

Based on those estimates, total permit fees for a typical urban ADU land roughly in the $4,600 to $7,300 range before impact fees. A rural detached ADU adds several thousand more in CUP fees.

Renting Your ADU

Long-term rentals (30 days or more) are allowed in both urban and rural zones. Rural detached ADUs still require the owner to live on the property.¹

Short-term vacation rentals (under 30 days) are different. Kitsap County adopted a vacation rental code in 2016 that requires conditional use permit approval for short-term rentals, similar to restrictions on bed-and-breakfast operations. The county has acknowledged the code is difficult to enforce and has been working on updates, so this area is still in flux. The code also specifically prohibits vacation rentals within the Gorst urban growth area between Sinclair Inlet and State Highways 3 and 16. If you’re building with short-term rental income in mind, confirm the current status with the Department of Community Development before committing.

Existing Unpermitted ADUs

If you already have an unpermitted ADU on a rural property, the county code offers a pathway to bring it into compliance rather than requiring demolition. Both attached and detached unpermitted ADUs located outside a UGA can apply for approval under Section 17.415.015, either by meeting current standards or by satisfying the criteria for existing unpermitted structures.¹ Formal approval matters for insurance, resale, and avoiding code enforcement action later.

1
Kitsap County Code. Kitsap County Code 17.415 – Allowed Use Standards
2
Washington State Legislature. RCW 36.70A.681
3
Kitsap County. Building on a Shoreline or a Steep Slope
4
Kitsap Public Health. Residential Building Clearances
5
Kitsap County Department of Community Development. Kitsap County 2025 Fee Schedule
6
Kitsap County Department of Community Development. Impact Fee Table
7
Kitsap County. Pre-Approved Accessory Dwelling Unit (ADU) Plans