What Homeowners Need to Know Before Starting Construction
Building a detached accessory dwelling unit (DADU) in Seattle is a smart way to expand your property’s function and value. Whether you’re creating space for extended family, a home office, or an income-generating rental, a DADU allows for flexibility without reducing the livability of your main home.
Seattle requires all DADU projects to meet specific zoning regulations, construction codes, and permitting standards to ensure legal use, safety, and long-term property value. As of 2025, several permitting rules have changed due to House Bill 1337, making it easier to add DADUs to both new and existing properties.
Key Takeaways
- Permit application requirements for DADUs in Seattle
- Recent changes under House Bill 1337 and what they mean
- SDCI process, required documents, and common pitfalls
- Fee updates and how to plan your budget
- How pre-approved DADU plans can reduce approval timelines
Using pre-approved DADU floorplans—such as the Schooner, Diana, or Anna—can simplify the review process and significantly reduce delays. These plans are designed to meet city standards and site conditions while providing a variety of layout options.
For homeowners and developers preparing to build a DADU in Seattle, understanding the updated permitting process is the first step toward a successful project. Reach out to our team to get expert guidance on how to get started.
Recent Updates to Seattle DADU Regulations (2025)
In 2025, Washington enacted several key changes to make DADU construction more accessible and efficient. These updates came through House Bill 1337, which aims to reduce permitting barriers and expand housing options across the state. Property owners in Seattle now have more flexibility to build additional living units on their lots.
Key Legislative Changes Under House Bill 1337
- Owner-occupancy rule eliminated: You no longer need to live on-site to build or rent out a DADU.
- Two ADUs now permitted per lot: You can build both an attached and detached ADU on the same property.
- More design freedom: The city has relaxed restrictions on setbacks, lot coverage, and maximum unit size.
These changes give homeowners and developers more ways to use their lots—especially in neighborhoods like Ballard, Beacon Hill, and Columbia City. You can now create more livable space without tearing down existing structures or rezoning.
Seattle’s Department of Construction & Inspections (SDCI) updated its permitting guidelines to reflect these reforms. The new rules clarify zoning and design requirements and help builders avoid time-consuming revisions during plan review.
For those looking to add rental units or family housing, DADUs now offer a more viable and cost-effective solution. The shift toward more flexible middle housing opens the door for smarter infill development throughout Seattle.
The Seattle DADU Permit Application Process
Before building a DADU in Seattle, you must complete the permit application process through the Seattle Department of Construction & Inspections (SDCI). Understanding the steps involved can help you prepare your documents, avoid delays, and ensure compliance with the latest building regulations.
Step-by-Step Overview
- Review zoning and lot eligibility: Use Seattle’s zoning map to confirm that your lot qualifies for a detached accessory dwelling unit.
- Prepare your plans and documents: Submit a full set of construction drawings, site plans, floorplans, and elevation views. Include your chosen DADU floorplan if using a pre-approved design.
- Apply online via the SDCI Permit Portal: The city requires digital submission through its permit system, which guides you through each step of the intake process.
- Wait for intake and plan review: After submission, your application will be reviewed by city staff. This includes checks for zoning compliance, construction safety, and environmental impact.
- Address corrections if needed: If the city identifies issues, you’ll receive feedback and must revise your plans before approval.
- Receive your approved permits: Once approved, you can schedule construction and begin building your DADU.
Working with a contractor who understands the Seattle DADU approval process can significantly reduce turnaround time. Our team handles submittals and coordinates directly with SDCI to keep projects moving smoothly.
Permit timelines vary based on project complexity and whether you’re using a pre-approved plan. Simple DADUs with compliant designs may move through faster, while custom projects or sites with environmental constraints may take longer to review.
It’s essential to start the permitting process early. This ensures your design meets all code requirements and avoids construction delays once you’re ready to break ground.
Zoning and Site Eligibility for Building a DADU
Before you invest time in design or submit a permit application, confirm that your property is zoned for a DADU. In most cases, lots within Seattle’s single-family and low-rise residential zones qualify. However, zoning overlays, environmental factors, and lot dimensions can affect eligibility.
Basic Eligibility Requirements
- Residential zoning: Most Neighborhood Residential (NR) and Urban Residential (UR) zones allow for DADUs. Verify your zoning through Seattle’s interactive zoning map.
- Minimum lot size: Seattle does not require a specific minimum lot size for DADUs, but the lot must meet access, utility, and layout standards.
- Lot width and access: Lots must have enough space to accommodate the structure with proper setbacks, and there must be access for emergency response vehicles.
Under the new Urban Residential (UR) zoning code, introduced in 2024 and expanded in 2025, additional flexibility has been granted for small-scale multi-unit development. The allowable Floor Area Ratio (FAR) increased from 0.9 to 1.2, giving property owners more design flexibility and room for larger units or dual ADUs.
Key Site Conditions to Consider
- Slopes or critical areas: If your lot includes steep slopes, wetlands, or other environmentally critical areas, you may need additional permits or engineering plans.
- Trees and landscaping: Seattle has tree protection rules that may affect placement of your DADU. Consult an arborist if you have significant trees on your lot.
- Existing structures: The location of garages, sheds, or driveways can impact where you’re allowed to build your DADU.
To help determine site eligibility, the SDCI offers early design guidance and zoning consultations. You can also view examples of DADUs built under similar conditions by browsing our DADU project gallery or reviewing real-world layouts on our DADU floorplans page.
Confirming zoning and site requirements at the start helps you avoid costly design changes later and ensures your project is feasible from both a legal and construction standpoint.
2025 DADU Permit Fees and Cost Expectations
In 2025, the City of Seattle updated its construction permit fees, including those for detached accessory dwelling units (DADUs). These changes reflect increased city service costs and new permitting standards introduced under House Bill 1337. Understanding these fees will help you budget more accurately for your project from the start.
Typical Permit Costs for a Seattle DADU
- Base permit fee: Approximately $2,800 for a 500 sq ft DADU
- Plan review fees: Often included in the base permit, but can vary if revisions are needed
- Additional fees: Utility connection charges, sewer capacity charges, energy code compliance fees, and impact fees (where applicable)
Permit costs can vary based on square footage, site complexity, and whether the project qualifies for expedited review. Using one of our pre-approved DADU floorplans can reduce plan review time and avoid unnecessary back-and-forth with the city, helping you save both time and money.
Seattle’s 2025 fee summary outlines specific cost categories for building permits, review services, and inspections. While exact amounts may vary based on project details, most standard DADU builds fall into a predictable fee range.
Tips to Manage Permit Costs
- Use pre-approved plans to minimize custom plan review fees
- Start with a full zoning and site review to avoid costly rework
- Work with a licensed builder familiar with the Seattle DADU permitting process
Proper budgeting during the planning phase ensures you account for all permitting expenses—not just construction costs. This allows you to avoid delays and unexpected fees once your project is underway.
The Fast Track: Using Pre-Approved DADU Plans
Seattle offers a major time-saving option for homeowners and developers through its library of pre-approved DADU plans. These designs meet the city’s building and zoning codes, allowing projects to bypass portions of the plan review process. This can significantly reduce approval times and simplify the entire permitting experience.
Benefits of Pre-Approved Plans
- Faster approval: Skip detailed plan review for structural compliance
- Lower design costs: Avoid hiring an architect to create plans from scratch
- Fewer revisions: City staff are already familiar with the designs
- Flexible layouts: Most pre-approved designs can be customized to fit your lot
At New Image Construction Management, we offer 17 customizable pre-designed DADU floorplans that meet Seattle’s current standards and site conditions. These designs include popular models like the Anita, Diana, and Schooner—each tailored to different lot sizes, privacy preferences, and accessibility needs.
By starting with a proven layout, you can minimize risk, reduce delays, and focus on site-specific customization instead of navigating complex code requirements from the ground up. This approach is ideal for property owners who want to stay on schedule and control costs.
Pre-approved plans don’t just benefit the permitting timeline—they also enhance clarity during construction, making it easier for contractors and inspectors to work efficiently. For many clients, this is the most efficient way to bring a DADU project to life in Seattle.
Common Pitfalls in the Permitting Process
While Seattle has made it easier to permit and build DADUs, many homeowners and developers still encounter preventable setbacks. Delays often result from incomplete applications, misunderstanding zoning limitations, or choosing designs that don’t align with local requirements. Knowing what to avoid can save weeks—or even months—on your project timeline.
Top Mistakes to Avoid
- Submitting incomplete documents: Missing site plans, floorplans, or elevation views can halt the intake process. Double-check submittal checklists before uploading your application.
- Overlooking zoning overlays: Some lots may fall under historic, environmental, or urban village overlays, which carry additional restrictions.
- Ignoring setback and height rules: Even under relaxed 2025 guidelines, DADUs must meet current limits for lot coverage and placement. Missteps often require costly redesigns.
- Failing to verify utility capacity: Water, sewer, and electrical capacity must be evaluated before approval. Delays often occur when this is not addressed early in the process.
- Not working with an experienced builder: A contractor unfamiliar with the Seattle DADU process may misinterpret codes or overlook critical submission requirements.
By planning carefully and avoiding these common pitfalls, you can increase the chances of receiving quick approval and keeping your construction schedule on track. Even small oversights—such as outdated forms or incorrect zoning assumptions—can cause weeks of delay.
If you’re unsure whether your site or design is compliant, our team offers early-stage consulting to review your plans before submission. We also handle full permitting services to reduce your administrative workload and ensure your application moves forward smoothly.
Partner with New Image for a Smoother Process
Building a DADU in Seattle doesn’t have to feel overwhelming. At New Image Construction Management, we’ve spent decades helping homeowners, investors, and developers navigate complex permitting requirements with clarity and confidence. Our experience working directly with the Seattle Department of Construction & Inspections (SDCI) allows us to anticipate challenges, avoid delays, and deliver results.
Whether you’re just beginning to explore your options or already have a design in mind, we offer full-service guidance from zoning review through construction. Clients choose us for our deep knowledge of Seattle DADU standards, commitment to quality, and proven success across a wide range of residential projects.
We also offer a library of pre-approved DADU floorplans that are customizable to meet your lot’s unique requirements. With our in-house design-build expertise, you can move from concept to construction with confidence and efficiency.
To see examples of our completed work, visit our DADU project gallery or explore current projects under construction. If you’re ready to move forward, our team is here to answer your questions and help bring your vision to life.
Request a Quote or Consultation
If you’re planning to build a DADU in Seattle and want expert guidance from start to finish, contact us today or call (206) 552-9448. We’ll walk you through every step of the permitting process and ensure your project starts off on solid ground.