ADU Questions Answered
Expert answers to 28 common questions about building an accessory dwelling unit in King County, Washington — costs, permits, zoning, timelines, types, and rental rules.
Category 1
Most single-family properties in King County are eligible to build an ADU. Eligibility depends on your lot size, zoning classification, setback requirements, and existing structures on the property. Washington State law has significantly expanded ADU rights for homeowners — making it easier than ever to add a backyard cottage, garage conversion, or basement unit. Contact us for a free feasibility check on your property.
In most King County jurisdictions, there is no longer a minimum lot size requirement for building an ADU, following Washington State ADU reform legislation. However, setback requirements, lot coverage limits, and impervious surface rules still apply and vary by city. Seattle, Bellevue, Redmond, and other cities each have their own specific rules — our builders know them all.
In King County, the maximum size for a detached ADU (DADU) is typically 1,000 square feet, though this varies by city. Attached ADUs and basement conversions may have different limits. Many cities also allow a second ADU (JADU) of up to 500 square feet in addition to a primary ADU. Our builders can clarify the exact limits for your specific property address.
ADUs are permitted across King County — including Seattle, Bellevue, Redmond, Renton, Kent, Kirkland, Issaquah, Sammamish, Burien, and unincorporated areas. Each city has its own permitting process, size limits, and setback rules. We serve all of King County and our builders are familiar with local requirements in every jurisdiction.
Owner-occupancy requirements for ADUs have been largely eliminated in King County and Washington State following recent legislation. Most jurisdictions no longer require you to live on the property to build or rent an ADU. However, rules vary by city — contact us to confirm the specific rules for your address.
In unincorporated King County (areas outside city limits), ADUs are permitted in residential zones. Setbacks are typically 5 feet from side and rear property lines for detached ADUs. Maximum size is generally 1,000 square feet or 40% of the primary dwelling's floor area. King County's Department of Local Services handles permitting for unincorporated areas.
Category 2
Yes — all ADUs in King County require building permits. This applies to detached ADUs, garage conversions, basement ADUs, and junior ADUs (JADUs). Permits ensure your ADU meets local building codes, which protects you legally and ensures the unit can be legally rented. Our builders handle the entire permit process from application to final inspection.
The ADU permit process in King County typically takes 2–6 months, depending on the city and complexity of the project. Seattle and Bellevue can take longer due to higher application volumes. Experienced ADU builders know how to prepare complete applications that minimize delays and avoid back-and-forth with plan reviewers.
A King County ADU permit application typically requires: a site plan showing lot layout and setbacks, architectural drawings (floor plan, elevations, sections), structural calculations, energy compliance documentation (Washington State Energy Code), and sometimes a geotechnical report. Requirements vary by city and ADU type. Licensed ADU builders prepare all permit documentation as part of the design-build process.
A DADU (Detached Accessory Dwelling Unit) is a specific type of ADU — a fully separate structure built in your backyard. "ADU" is the broader term covering all types: detached ADUs (DADUs or backyard cottages), attached ADUs, garage conversions, basement conversions, and junior ADUs (JADUs). In Seattle, DADU is the commonly used term for a backyard cottage.
For simpler ADU projects — like garage conversions or small detached units — a licensed architect is not always required. However, most cities require stamped structural drawings, which need a licensed structural engineer. For detached ADUs over 500 square feet, working with an experienced design-build team typically produces better results and fewer permit delays.
Category 3
ADU construction costs in King County typically range from $150 to $400 per square foot, depending on the type, finishes, and site conditions. A detached backyard ADU might cost $200,000–$400,000+. A garage conversion usually runs $80,000–$150,000. A basement ADU often costs $60,000–$120,000. Permit fees, design fees, and utility connections add additional costs beyond hard construction.
Yes — home equity is the most common ADU financing source in King County. Options include home equity loans (HEL), home equity lines of credit (HELOC), cash-out refinancing, and construction loans. Some homeowners also use Fannie Mae HomeStyle renovation loans. Washington State has ADU pre-approved plan programs that can reduce design costs and speed up permitting.
Washington State and several King County cities have programs to support ADU development, particularly for affordable housing purposes. Some utilities offer rebates for energy-efficient construction methods. Some cities offer expedited permitting or fee waivers for ADUs committed to affordable rental rates. Our team can help identify available incentives for your specific project and location.
Rental income from King County ADUs can be significant. Average ADU rents in Seattle range from $1,500–$2,800/month depending on size and location. A $250,000 ADU renting for $2,000/month provides roughly a 9.6% annual gross yield. ADUs also increase your property's resale value — typically adding $150,000–$300,000 to a King County home's market value.
This varies by builder. Some design-build ADU contractors include permit fees in their all-in price; others quote construction costs separately from permit, design, and utility connection fees. Always request a fully itemized estimate that clearly separates hard construction costs, soft costs (permits, design, engineering), and utility connection fees before signing any contract.
Category 4
A detached ADU (backyard cottage or DADU) is a fully independent dwelling unit built as a separate structure in your backyard. It has its own entrance, kitchen, bathroom, and living space. DADUs offer maximum privacy for both you and a tenant and typically command the highest rental rates. They are ideal if you have sufficient yard space and want to maximize rental income or create private housing for family.
A garage conversion ADU transforms an existing attached or detached garage into a permitted, livable dwelling unit. The existing structure reduces construction costs significantly compared to building from scratch. Typical work includes insulating walls and ceiling, adding plumbing, upgrading electrical, installing egress windows, and adding proper heating. Garage conversions are one of the most cost-effective ADU types in King County.
A basement ADU converts an existing basement or lower level into a permitted, code-compliant dwelling unit. This typically involves adding egress windows, ensuring minimum ceiling height (7 feet), creating a separate entrance, and adding a kitchen and bathroom. Basement ADUs are among the most affordable ADU types since the foundation and structure already exist — ideal for multi-generational living or supplemental rental income.
A Junior ADU (JADU) is a small dwelling unit of up to 500 square feet created within the walls of an existing single-family home — typically from a bedroom, bonus room, or attached garage area. JADUs must include their own exterior entrance and an efficiency kitchen. They are the least expensive ADU type to create. In many King County cities, a JADU can be combined with a primary ADU to create two additional units on the property.
Junior ADUs (JADUs) are generally the least expensive to build, often $30,000–$80,000, since they convert existing interior space with minimal structural work. Basement ADUs are the next most affordable ($60,000–$120,000), followed by garage conversions ($80,000–$150,000). Detached ADUs (backyard cottages) are the most expensive ($150,000–$400,000+) but also generate the highest rental income and property value increase.
Category 5
Total ADU project timelines in King County typically range from 9 to 18 months from start to move-in. Design and permit preparation takes 1–3 months. Permit review and approval takes 2–6 months depending on the city. Construction takes 3–6 months for a detached ADU, and 2–4 months for a garage conversion or basement ADU. Working with an experienced local ADU builder significantly reduces delays at every stage.
Building a King County ADU involves: (1) Feasibility assessment — confirm eligibility, zoning, and setbacks. (2) Design — hire an architect or design-build contractor to create plans. (3) Permit application — submit drawings to your city. (4) Permit review — respond to any comments. (5) Permit issuance. (6) Construction — begin building with your licensed contractor. (7) Inspections — pass required city inspections. (8) Certificate of Occupancy — receive your CO and begin renting or using the unit.
Yes. Washington State requires all ADU construction to be performed by a licensed contractor registered with the Department of Labor & Industries (L&I). Contractors must carry both general liability insurance and workers' compensation coverage. Hiring an unlicensed contractor could result in permit issues, code violations, and difficulty selling your home. Always verify your builder's L&I registration number before signing a contract.
Category 6
Yes. Once your ADU receives a Certificate of Occupancy, you can legally rent it as a long-term rental unit anywhere in King County. Rental income from ADUs is treated as ordinary income for federal and state tax purposes. Some cities require a rental registration or business license — check your city's specific requirements. Washington State's landlord-tenant laws apply to all ADU rentals.
Short-term rental (STR) rules for ADUs vary significantly across King County cities. Seattle requires a Short-Term Rental Operator License and limits STRs in certain zones. Bellevue, Redmond, and other cities have their own STR regulations. Some HOAs also prohibit short-term rentals entirely. Before counting on STR income, verify the current rules in your specific city — they change frequently.
Yes. Building an ADU increases your property's assessed value, which raises your annual property taxes. The amount of increase depends on the ADU type, size, and your current assessed value. King County assessors typically reassess the property after a permit is issued. However, the rental income from a well-built ADU almost always more than offsets the property tax increase — often by a factor of 5x or more.
In most King County jurisdictions, ADUs cannot currently be sold separately (subdivided/condominiumized) from the primary dwelling under traditional zoning. However, Washington State has passed legislation enabling ADU condominiumization in some circumstances, and this area of law is actively evolving. Consult a real estate attorney if separate ownership is your ultimate goal.