How Much Does It Cost to Build an ADU or DADU in Seattle?
The short answer is that an attached ADU may start around $150,000, while a new detached DADU can easily become a $350,000 to $600,000+ project. The real answer depends on the property, the design, the utilities and what surprises are hiding below the surface.
A DADU can add flexibility and value, but the numbers need to work before the project starts.
I wish there were one clean price for building an ADU or DADU in Seattle. There isn’t.
A straightforward basement conversion is a very different project from building a detached two-story cottage behind a house on a steep lot. One may use existing walls and utilities. The other may need a foundation, trenching, upgraded electrical service, drainage work, tree review and a custom design.
That is why the cheapest estimate is rarely the number you should use to make the decision. You need the total project cost, including the expenses that show up before construction, outside the contractor’s original bid and after the unit is finished.
Seattle ADU and DADU Costs at a Glance
These are broad planning ranges, not quotes. Seattle construction costs can move quickly, and two similar-looking properties can produce very different bids.
Actual costs depend on design, site conditions, utilities, permits, finishes, financing and contractor pricing. Difficult sites and high-end projects can exceed these ranges.
What This Cost Guide Covers
Attached ADU vs. Detached DADU Costs
An attached accessory dwelling unit is inside or connected to the main house. A basement apartment, converted garage or separate suite may reuse parts of the existing structure and nearby utilities.
Planning range: roughly $150,000 to $300,000+, depending on what already exists and what must be corrected.
A detached accessory dwelling unit is a separate backyard cottage or secondary home. It typically requires a new foundation, full structure, utility connections and more site work.
Planning range: roughly $350,000 to $600,000+, with custom or difficult projects potentially costing more.
Where the Money Actually Goes
The construction bid is only one part of the budget. A realistic Seattle ADU or DADU plan should account for the full project.
| Cost category | Broad planning range | What it may include |
|---|---|---|
| Feasibility and due diligence | $2,500–$10,000+ | Property research, zoning, survey work, tree review, utility research and early design input. |
| Design and engineering | $15,000–$60,000+ | Architecture, structural engineering, energy compliance, permit drawings and revisions. |
| Permits and review | $8,000–$30,000+ | Building and trade permits, review fees, corrections and related documentation. |
| Construction | $150,000–$500,000+ | Demolition, foundation, framing, roofing, windows, insulation, electrical, plumbing, HVAC and finishes. |
| Utilities and site work | $25,000–$100,000+ | Sewer, water, power, drainage, trenching, grading, access, fencing and landscaping. |
| Contingency | 10%–20% | Unknown conditions, scope changes, price changes and permit-related surprises. |
These figures are educational planning ranges only. Get property-specific estimates from qualified Seattle professionals before making a financial decision.
Seattle Rules Can Change the Budget
Seattle has expanded ADU flexibility. Current city guidance generally allows two ADUs in addition to a primary home on residential lots, and the general size limit has increased to 1,000 square feet. That does not mean every lot can support the same design.
Setbacks, height, lot coverage, trees, utilities, shoreline rules, environmentally critical areas and the existing buildings still matter. The City also requires permits and code compliance. A unit is not legal simply because it has a kitchen, bathroom and separate entrance.
Start with the Seattle SDCI accessory dwelling unit guide and the City’s ADUniverse planning tools. Washington’s broader ADU requirements are found in RCW 36.70A.681.
Will the Rent and Property Value Justify the Cost?
This is where the conversation gets real. A DADU may produce rental income and make the property more attractive to certain buyers, but neither outcome is guaranteed.
As a broad starting point, some attached Seattle ADUs may rent in the $1,500 to $3,000+ range, while private detached units may rent around $2,500 to $4,500+ depending on the neighborhood, size, condition, parking, privacy and amenities. Those are planning ranges. Use current rental comparables for the specific property before relying on the income.
A simple way to test the numbers
Total the entire project
Include design, permits, construction, utilities, financing, contingency and carrying costs.
Estimate realistic rent
Compare similar units nearby. Do not use the highest furnished short-term rental you can find.
Subtract operating expenses
Allow for vacancy, maintenance, insurance, taxes, utilities, management and future repairs.
Consider resale value
A permitted, useful unit may add value, but buyers and appraisers will still judge the quality, layout and local demand.
Simple yield formula: Net annual rental income divided by total project cost. That gives you a starting yield before appreciation, taxes and financing. If the deal only works with perfect rent, no vacancy and no cost overruns, it probably does not work.
How Do Seattle Homeowners Finance an ADU or DADU?
Flexible access to home equity, usually with a variable rate. The payment can change, and the available equity must support the project.
A fixed second mortgage may offer more predictable payments when the project budget is clearly defined.
This can unlock equity, but replacing a favorable first mortgage may make the overall cost much higher.
Some larger projects use construction loans with draws, inspections, underwriting requirements and strict budgets.
Talk with a lender before paying heavily for design. Future rent may or may not be counted the way you expect, and the completed ADU value may not equal the cost of construction.
Can a Pre-Approved DADU Plan Save Money?
Seattle offers a gallery of pre-approved DADU designs. These plans have already been reviewed for certain building, land-use and energy-code requirements, which can make the design and review path more predictable.
Pre-approved does not mean pre-approved for your lot. You still need site-specific work involving the foundation, utilities, drainage, trees, access and other property conditions. You may also need to license the plan from its designer.
Explore the current options in the City’s pre-approved DADU gallery.
Does Building an ADU or DADU Make Sense?
Sometimes the answer is absolutely yes. The unit may house a parent, create dependable rental income, give an adult child some independence or make the property work for the next phase of life.
Sometimes the answer is no. The project may consume too much equity, damage the main home’s privacy, remove valuable yard space or cost more than the market will recognize at resale.
The property is feasible, the budget has breathing room, the unit serves a clear long-term purpose and the numbers still work with conservative assumptions.
The plan depends on maximum rent, a perfect construction schedule, no utility surprises or recovering every construction dollar immediately at resale.
There is also a third option: sell the current property and buy one with an existing permitted ADU or better development potential. Building is not automatically the best move just because it is possible.
Common Seattle ADU and DADU Cost Questions
How much does a DADU cost in Seattle?
A new detached DADU may cost roughly $350,000 to $600,000+ as a broad planning range. Custom design, difficult access, utilities, slopes, drainage, trees and premium finishes can push the total higher.
How much does an attached ADU cost?
An attached conversion may fall around $150,000 to $300,000+, but the existing home controls the price. Structural work, ceiling height, moisture, egress, plumbing and code upgrades can materially increase the cost.
Does a DADU increase home value?
It can. Permitted status, construction quality, privacy, parking, rental potential and neighborhood demand all influence how buyers and appraisers respond. The added value is not guaranteed to equal the construction cost.
Can I use projected rent to qualify for financing?
Possibly, but lenders have different programs and underwriting rules. Speak with a lender who understands ADU projects before relying on projected income.
Is a pre-approved Seattle DADU plan automatically permitted?
No. The design may be pre-approved, but your property still requires site-specific review, permits and inspections.
Run the Numbers Before You Build
An ADU or DADU can be a fantastic addition to a Seattle property. It can also become a very expensive lesson if the plan starts with optimistic rent and an incomplete budget.
Start with the property. Verify what is possible. Price the entire project. Then look at rent, financing, resale value and how the unit will actually improve your life or investment.
The Madrona Group can help with the real estate side of that decision, including buyer demand, rental context, resale strategy and whether buying or selling may make more sense than building.
Related Seattle Real Estate Resources
Keep researching before you build, buy or sell.
Sources and Important Notes
Official starting points include the Seattle Department of Construction and Inspections, ADUniverse and Washington’s ADU statute.
This article is for general education and real estate planning. Costs, rents, financing and resale outcomes vary. Codes and permit requirements can change. Confirm the current rules and property-specific details with Seattle SDCI and qualified design, construction, engineering, lending, insurance, tax and legal professionals.
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2 Comments on “How Much Does It Cost to Build an ADU or DADU in Seattle?”
This is garbage. DADU does nothing but piss off your neighbor, invade their privacy and increase tax revenue for the city. Screw anyone who builds a DADU.
Hey, don’t shoot the messenger. I don’t have a DADU. I can understand your frustration, and it is the solution that our government has come up with trying to solve the inventory and affordability problems.