As housing demand grows in Washington State, more homeowners are looking for ways to maximize their property’s value and functionality. One of the best ways to do this is by building a Detached Accessory Dwelling Unit (DADU), a self-contained secondary residence separate from the main home.
DADUs, also known as backyard cottages, guest houses, or carriage houses, offer flexible living spaces for rental income, multigenerational housing, or home offices. But before you start building, it’s essential to understand the costs, financing options, and zoning laws in Washington State to ensure compliance and a smooth construction process.
In this guide, we’ll cover:
- How much it costs to build a DADU in Washington State
- Available financing options to fund your DADU project
- Washington State’s ADU zoning laws and local regulations
What is a Detached Accessory Dwelling Unit (DADU)?
A Detached Accessory Dwelling Unit (DADU) is a separate, self-contained residential unit located on the same lot as a single-family home. Unlike an Attached ADU (AADU), which is connected to the main house (such as a basement apartment), a DADU stands alone and typically includes a kitchen, bathroom, and living space.
Top Benefits of Building a DADU
Detached Accessory Dwelling Units (DADUs) are more than just backyard projects, they’re smart, flexible spaces that add long-term value and functionality to your property. Here’s why more Washington homeowners are investing in DADUs:
- Generate Rental Income – Rent it out long-term or as an Airbnb for passive income.
- Multigenerational Housing – Provide independent living for elderly parents or adult children.
- Home Office or Studio – Create a dedicated workspace separate from the main home.
- Increase Property Value – Boost your property’s resale appeal and marketability.
How Much Does It Cost to Build a DADU in Washington State?
The cost of building a Detached ADU in Washington varies depending on location, design, materials, and site conditions. On average, DADU construction costs range from $250,000 to $450,000 in Washington State.
Estimated Cost Breakdown
Factors Affecting Cost:
✔ Size & Design Complexity – Larger or custom-designed units cost more.
✔ Building Materials – High-end finishes and sustainable materials increase costs.
✔ Site Conditions – Sloped lots or difficult terrain may require extra preparation.
✔ Labor Costs – Prices vary by region and contractor expertise.
Financing Options for DADUs in Washington State
Building a DADU is a major investment, but homeowners have multiple financing options to make the project more affordable:
1. Home Equity Loans
A home equity loan allows you to borrow against the equity in your property. These loans offer fixed interest rates and structured repayment terms, making them a popular option.
2. Construction Loans
A construction loan provides short-term financing for the building phase and converts into a permanent mortgage upon project completion.
3. Cash-Out Refinancing
With cash-out refinancing, you replace your existing mortgage with a higher-value loan, using the extra funds for construction.
4. Personal Loans
For smaller ADU projects, some homeowners opt for personal loans, though they typically have higher interest rates.
Washington State Guidelines for DADUs
Washington State has enacted legislation to promote the development of Accessory Dwelling Units (ADUs), including DADUs:
- Minimum Allowance: Cities within Urban Growth Areas (UGAs) are mandated to allow at least two ADUs on all lots that permit single-family homes.
- Size Requirements: Local governments may not require ADUs to be smaller than 1,000 gross square feet in size. org
- Parking: Additional parking requirements for ADUs are generally restricted, especially for properties near major transit stops.
- Owner-Occupancy: The state has removed the owner-occupancy requirement, allowing both the primary residence and the ADU to be rented out.
Local Zoning Regulations
While the state provides a framework, local municipalities have the authority to implement specific regulations:
- Size and Design: Local governments may impose design standards to ensure ADUs are compatible with neighborhood aesthetics.
- Setbacks and Height: Regulations concerning setbacks from property lines and height restrictions can vary by locality.
- Permitting: The process and fees for obtaining permits differ across cities and counties.
Examples of Local Regulations
- Seattle: Allows up to two ADUs per lot (one attached and one detached), with no owner-occupancy or off-street parking requirements. The maximum size for a DADU is 1,000 square feet in neighborhood residential zones.
- Bothell: Permits one ADU per lot, with size limitations and specific height restrictions. An additional parking space is required, and owner occupancy in one of the units is mandatory.
- Burien: Allows one attached and one detached ADU per lot, with specific size restrictions and parking requirements.
By understanding both state guidelines and local regulations, you can effectively plan and construct a DADU that meets legal requirements and enhances your property’s value.
Why Choose Goodwin Construction for Your DADU Project?
At Goodwin Construction, we specialize in Detached ADU construction throughout Washington State, ensuring that your project meets local zoning regulations, budget, and design vision.
Why Work With Us?
✔ 25+ Years of Experience – Proven expertise in residential construction.
✔ Custom DADU Design & Construction – Tailored to your needs and property.
✔ Knowledge of Washington’s ADU Laws – Ensuring compliance and smooth approvals.
✔ Transparent Pricing & Financing Assistance – Helping you maximize your investment.
Ready to Build Your DADU? Contact Us Today!
Building a Detached Accessory Dwelling Unit in Washington State is a smart investment that enhances property value, generates rental income, and expands housing options.
Frequently Asked Questions
What is a DADU and how does it differ from an ADU?
A DADU (Detached Accessory Dwelling Unit) is a self-contained living space separate from your main home, usually in a standalone structure. An ADU (Accessory Dwelling Unit) includes attached or converted units. Washington state law allows both, but local regulations may differ.
How big can a DADU be in Washington State?
In many jurisdictions, a DADU is limited to about 1,000 sq ft maximum in single-family zones. For example, in Seattle the standard cap is 1,000 sq ft (or 650 sq ft in low-rise zones).
Can I build two ADUs (or DADUs) on one lot in Washington State?
Yes. Under state law (RCW 36.70A.680/681), fully-planning cities must allow up to two ADUs on lots that permit single-family homes, this can include one DADU and one AADU (attached) or two AADUs.
Do DADUs require owner occupancy in Washington State?
In many Washington jurisdictions, the owner-occupancy requirement has been eliminated, meaning the owner does not have to live on site or in one of the units. Check local rules for details.
What are typical costs for building a DADU in Washington State?
What permits and approvals are required for a DADU?
You’ll need a building permit, possibly electrical/plumbing permits, and compliance with zoning codes (setbacks, height, lot size). For example, Snohomish County provides detailed checklists for DADU permit submission.
Can a DADU be used for rental income in Washington State?
Yes. Many homeowners use a DADU for long-term rental or short-term lease, provided it meets local occupancy and zoning rules. State law allows the main home and DADU(s) to be rented unless the local jurisdiction restricts.
How do I finance a DADU in Washington State?
What factors affect DADU cost and feasibility in Washington?
Why choose Goodwin Construction for my DADU project in Washington State?
Goodwin Construction knows Washington’s ADU/DADU codes, permitting pathways and construction best-practices. We make the process clearer, budget-transparent and build-efficient so your DADU becomes a value-adding asset.
