How Much Does it Cost to Build an ADU in Seattle? (2026 Guide)

Building an Accessory Dwelling Unit (ADU) is one of the most popular home improvement projects in the Pacific Northwest. Whether you call it a backyard cottage, a DADU (Detached Accessory Dwelling Unit), or a mother-in-law suite, Seattle homeowners are finding new ways to use their lot space.

However, the financial landscape has shifted. In 2026, the cost to build an ADU in Seattle typically ranges from $100,000 to over $300,000, depending heavily on size, scope, and whether it’s attached or detached. Complex sites (steep slopes, difficult access, long utility runs, premium finishes) can push totals higher.

For a widely used construction cost reference, many professionals rely on RSMeans (by Gordian) (see: RSMeans 2026 Cost Data Books).

This guide breaks down where your budget goes, from the first permit application to the final coat of paint.

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Understanding Seattle ADU Costs by Type

Not all ADUs are created equal. In Seattle, the terminology matters for both your budget and your permit application. The Seattle Department of Construction and Inspections (SDCI) summarizes the two main categories in its Accessory Dwelling Units overview.

1. Attached ADU (AADU)

• Typical Cost: $100,000 – $225,000
• What it is: A basement conversion, garage conversion, or an addition attached to the main house.
• Cost Drivers: Because the structure (foundation, roof) often already exists, costs can be lower. However, retrofitting plumbing, electrical, ventilation, and meeting egress requirements (especially in older basements) can raise costs quickly.

2. Detached ADU (DADU)

• Typical Cost: $250,000 – $400,000+
• What it is: A standalone backyard cottage.
• Cost Drivers: You’re essentially building a small house: new foundation, new utility connections (sewer/water), full framing, full mechanical systems, and full finishes.

Per-square-foot estimates can be useful for early planning, but your final price will depend heavily on site conditions and scope. If you want an evergreen RSMeans source (rather than a single book page), see RSMeans Data Services.

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Detailed Cost Breakdown: Where Does the Money Go?

Many homeowners experience “sticker shock” when they see a quote. To help you plan your Seattle remodel budget, here’s how costs often break down for a typical project in neighborhoods like Ballard or West Seattle.

The “Invisible” Costs: Site Work

Seattle’s topography is beautiful and expensive. If your lot in Queen Anne or Capitol Hill has a steep slope or limited access for machinery, site preparation can increase dramatically. Stormwater and drainage requirements can also add significant scope depending on what you’re building and how much impervious surface you’re adding.

Helpful references:

• SDCI Stormwater Code
• SPU Sewer & Drainage Construction Resources

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How Much Are Permits and Soft Costs?

Before you break ground, you must navigate design, engineering, and permits. “Soft costs” often total 10–15% of the overall project.

Design Fees vs. Pre-Approved Plans

You generally have two options:

1. Custom Architect
   Typical Cost: $15,000 - $30,000+
   Best for unique lots, complicated constraints, or custom layouts.

2. ADUniverse Pre-Approved Plans (City of Seattle)
   Seattle offers a catalog of pre-approved detached ADU plans that can reduce design costs and shorten permit review time:

   • ADUniverse home
   • Pre-approved plan gallery
   • ADUniverse step-by-step guide

Utility Hookups and Sewer-Related Costs

For detached ADUs, utility work can be a major cost driver, including side sewer work and capacity charges.

• King County wastewater capacity charge
• SPU side sewer permits

Note: As of October 1, 2025, SPU conducts sanitary/wastewater plan review, side sewer permitting, and side sewer inspections. See SDCI’s notice about Side Sewer moving to SPU.

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The Impact of Seattle ADU Rules

Seattle has updated rules in recent years to encourage added housing capacity. Highlights commonly referenced by homeowners include:

• Two ADUs per lot: In many cases, Seattle allows multiple ADUs on a single lot (for example, one attached and one detached), subject to code limits and development standards.
• No new parking requirement in many cases: ADUs are generally not required to add off-street parking.
• No owner-occupancy requirement: Owner occupancy requirements have been removed.

Start here for official details and links to requirements:

• SDCI ADU overview
• SDCI ADU code updates
• Seattle Municipal Code (Municode)

These changes can make the process smoother, but demand can still keep labor prices high across the region.

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Is Building an ADU Worth the Investment?

Despite the upfront cost, an ADU can be a strong long-term move for both flexibility and value.

Rental Income Potential

Depending on neighborhood, finishes, and layout, a 1-bedroom DADU can often rent for $1,800 to $2,500 per month. Actual rent will vary based on location, access, parking, and the overall quality of the unit.

Resale Value

ADUs often increase marketability and can improve resale value, though it may not be dollar-for-dollar in an appraisal. For broader remodeling ROI benchmarks, see the Cost vs. Value Report.

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How Long Does it Take to Build?

A typical Seattle DADU timeline in 2026 looks like:

1. Feasibility & Design: 1–2 months
2. Permitting (SDCI): 2–4 months (often faster with pre-approved plans)
3. Construction: 4–6 months

Total Timeline: Expect 7 to 12 months from idea to move-in day.

Pro Tip: Seattle winters are wet. Starting your foundation work in July or August can help you get the structure “dried in” before heavy fall rains.

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Financing Your Backyard Project

Most homeowners do not pay $300,000 cash. Common financing options include:

• HELOC (Home Equity Line of Credit): Uses your existing home’s equity.
• Construction Loan: Short-term financing that may convert to a mortgage.
• Renovation Loan (FHA 203k): Can bundle renovation costs with a mortgage (eligibility varies).

If you want a rough monthly payment estimate, try our Project Cost Calculator to run the numbers.

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FAQs About Seattle ADUs

Can I sell my DADU separately from my house?

Separate sale is typically done through a condominium structure and requires specialized legal work. Helpful references:

• Washington State Condominium Act (RCW 64.34)
• WA Dept. of Commerce ADU info
• SDCI condominium conversions (general)

Do I need a separate electrical meter?

You don’t have to, but it’s often recommended for long-term rentals to avoid utility disputes and simplify billing. A service upgrade may be required depending on your existing panel capacity.

How close to the property line can I build?

Setbacks and development standards vary by zone, lot conditions, and whether the lot line abuts an alley. Always confirm current requirements with:

• SDCI ADU overview
• Seattle Municipal Code (Municode)

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Ready to Plan Your Backyard Cottage?

Building an ADU is a major project, but it can transform your property’s value and flexibility. Whether you need space for family or want a rental income stream, getting an accurate estimate is the first step.

Check out our ADU Services Page to view our portfolio, or contact us today to discuss feasibility for your specific Seattle lot.

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Sources & References

1. Seattle Department of Construction and Inspections (SDCI): Accessory Dwelling Units
2. ADUniverse (City of Seattle)
3. ADUniverse: Pre-Approved Plan Gallery
4. ADUniverse: Step-by-Step Guide
5. RSMeans 2026 Cost Data Books
6. RSMeans Data Services (Gordian)
7. SDCI Stormwater Code
8. Seattle Public Utilities: Sewer & Drainage Construction Resources
9. Seattle Public Utilities: Side Sewer Permits
10. SDCI: Side Sewer to SPU (Oct 1, 2025)
11. King County: Wastewater Capacity Charge
12. SDCI: ADU Code Updates
13. Seattle Municipal Code (Municode)
14. Remodeling Magazine: Cost vs. Value Report
15. Washington State Legislature: RCW 64.34 Condominium Act