The real cost of a build-to-sell DADU isn’t just construction — it’s construction plus legal separation (condoization), closing costs, and a sales commission. The only number that actually matters is whether that full total comes in meaningfully below what the finished unit will realistically sell for. Budgeting off the construction number alone is the most common way a build-to-sell project ends up with a thinner margin than expected.
DADU Construction Cost in Seattle: The Baseline
Across current Seattle-area projects, a detached ADU typically runs somewhere in the $300 to $500+ per square foot range all-in, with a full backyard cottage project commonly landing between roughly $250,000 and $500,000 depending on size, finish level, and site conditions. Smaller units and garage conversions can come in lower; larger, two-story units or projects on difficult lots push toward the higher end. Permit fees alone typically add tens of thousands of dollars on top of hard construction costs, and soft costs — design, engineering, permitting — usually run another 10-15% of the total project.
This is the baseline number most people start with, and it’s the right number if you’re comparing builders or scoping the project. But it’s not the number that tells you whether a build-to-sell project pencils out.
For a full line-item breakdown of construction costs — design, permits, materials, and finishes — see our complete guide on how much an ADU costs to build.
The Costs a Rental Budget Doesn’t Have to Account For
If you’re building to rent, construction cost plus ongoing carrying costs is roughly the whole picture. If you’re building to sell, there’s a second layer of cost that a rental budget simply doesn’t need:
- Condoization costs — a licensed survey, an attorney to draft the condominium declaration, formation of the required homeowners association, and county recording fees. This is a real, multi-party legal process, not a formality.
- Closing costs on the sale — in Washington, the seller typically pays the Real Estate Excise Tax (REET) at closing. As of 2026, the state REET is graduated: 1.1% on the portion of the sale price up to $525,000, 1.28% up to $1.525 million, and higher rates above that — plus a local REET, commonly around 0.5% in Seattle and most surrounding jurisdictions. Escrow and title insurance fees apply on top of that.
- Real estate commission — selling through an agent typically runs in the neighborhood of 5-6% of the sale price, split between the buyer’s and seller’s agents, though this is negotiable and varies by brokerage.
- Holding costs during marketing — loan interest, utilities, insurance, and property tax for however long the unit sits on the market before closing. Even a few extra months here adds up.
None of these show up in a typical “cost to build an ADU” estimate, because that number is written for someone who’s keeping the unit. For a build-to-sell project, they’re not optional line items — they come directly out of your proceeds.
For the step-by-step mechanics of the condoization process specifically, see our guide on DADU condoization in Seattle.
DADU vs. ADU Cost Comparison — Which Makes Sense to Build to Sell?
An attached ADU is usually cheaper to build than a detached DADU, since it can share walls, roofline, or foundation with the existing house. If your only question were “which costs less to build,” the attached option often wins.
But cost per square foot is the wrong comparison for a build-to-sell decision. What matters more is sellability — and a detached structure is the one that separates cleanly into its own condominium unit. Because a DADU is physically independent from the main house, it can go through the condoization process described above and become a genuinely separate, independently titled property. An attached ADU, sharing structural elements with the primary residence, generally doesn’t separate the same way, which limits its usefulness as a standalone asset to sell.
In other words: for a rental strategy, the cheaper-to-build attached option might make more financial sense. For a build-to-sell strategy, the detached DADU’s higher construction cost is often the price of the very thing that makes the strategy possible in the first place.
For the general trade-offs between the two ADU types, see our guide on ADU vs. DADU: maximizing your property potential.
What Does a DADU Actually Sell For in Seattle?
A rental ADU’s value is usually estimated off rent. A condoized DADU sells differently — more like a small standalone home, priced off neighborhood comparable sales rather than projected rental income — and because separate DADU sales are still a fairly new option in Washington, comparable sales data is thinner than what you’d find for a typical home resale.
We go deep on what actually drives DADU sale prices — location, layout, privacy from the main house, and what real 2026 Seattle sales have looked like so far — in our dedicated guide on DADU resale value. The short version for budgeting purposes: don’t plug a general online estimate into the break-even math below. Get a current, neighborhood-specific opinion of value from a local agent before you commit construction dollars to a sale-based exit.
Is Building a DADU to Sell Worth It? A Break-Even Framework
Rather than a single yes-or-no answer, it’s more useful to run your own numbers through a simple framework:
- Total build-to-sell cost = construction cost + condoization/legal costs + closing costs (REET, escrow, title) + real estate commission + holding costs during marketing
- Realistic sale price = based on actual local comparable sales, not a general online estimate
- Margin = realistic sale price − total build-to-sell cost
A few things worth keeping in mind as you fill in your own numbers:
- Get real quotes for each category — a contractor’s construction estimate, a condo attorney’s fee estimate, and a local agent’s opinion of value — rather than relying on national averages, which tend to understate what things actually cost in Seattle specifically.
- Build in a timeline buffer, not just a cost buffer. Condoization and permitting can take longer than expected, and every extra month adds holding costs that erode your margin even if construction itself came in on budget.
- Factor in the time value of your capital, not just the dollar totals. A build-to-sell project typically ties up significant capital for a year or more, with real execution risk along the way — market conditions can shift, and condoization delays are common enough to plan around rather than hope against.
A thin margin on paper, once you account for a realistic timeline and the possibility of delays, often isn’t worth the risk compared to a rental strategy with steadier, if slower, returns. A healthy margin — one that still holds up after a longer-than-expected timeline — is what actually makes a build-to-sell DADU worth pursuing.
For how to structure financing specifically around this kind of sale-based exit — including how to budget for delays in the condoization process — see our guide on financing a build-to-sell DADU.
FAQ
How much does it cost to build a DADU in Seattle? Current Seattle-area projects generally run $300 to $500+ per square foot all-in, with a typical detached DADU project landing somewhere between $250,000 and $500,000 depending on size and finish level. See our full construction cost breakdown for the detailed line items.
What does it cost to build a DADU specifically to sell it? Beyond construction, budget for condoization (survey, attorney, HOA formation, recording fees), closing costs on the sale (REET, escrow, title insurance), real estate commission, and holding costs while the unit is on the market. These add meaningfully to the construction number alone.
How much is a backyard cottage in Seattle in 2026? Most detached backyard cottages in Seattle fall in the $250,000 to $500,000 range to build, with per-square-foot costs generally between $300 and $500 or more depending on size and finishes.
What’s the construction cost per square foot for a DADU? Current Seattle-area ranges generally fall between $300 and $500+ per square foot, though smaller units and simpler finishes can come in lower, and larger or more complex projects can run higher.
What’s the total cost to build and sell a DADU, including fees? Add condoization costs, sale closing costs (Washington’s graduated REET plus local REET, escrow, and title insurance), and a typical 5-6% real estate commission to your construction total to get a realistic full-cycle cost.
Is building a DADU to sell actually worth it? It depends on your specific numbers — construction cost, condoization and closing costs, and what comparable units are actually selling for in your neighborhood. Run the break-even framework above with real quotes rather than relying on a general answer.
How does DADU cost compare to attached ADU cost? An attached ADU is usually cheaper to build, but a detached DADU is the type that can be condoized and sold as a fully separate property. For a build-to-sell strategy, sellability tends to matter more than the raw construction cost difference.
How much does a DADU sell for in Seattle? It depends heavily on neighborhood, size, and finish quality. For current figures and the factors that drive DADU sale prices specifically, see our dedicated guide on DADU resale value.
