Seattle ADU Directory Verified permit records
← Seattle ADU builders

What does an ADU cost in Seattle? A practical guide

Every homeowner researching an ADU asks the same first question: what’s this actually going to cost? The honest answer is that nobody can tell you a real number except a licensed contractor looking at your specific lot — but this guide can tell you what’s knowable in advance, what typically drives the biggest swings in price, and which fees are fixed, published, and verifiable versus which numbers are genuinely just market estimates.

Read this before anything else. Nothing on this page is a quote, an estimate for your specific project, or financial or tax advice. Construction costs vary enormously by site conditions, design choices, and who you hire, and permit fees and tax rules change from year to year. Where this guide cites a specific published fee — a permit cost, a utility charge — that number comes from a primary government source and is dated. Where this guide discusses total project costs, those figures are a synthesis of typical ranges reported across the Seattle market, not independently verified data the way the permit records elsewhere on this site are, and should be treated as a rough starting orientation only. Before you commit money: get multiple itemized bids from licensed contractors, confirm current fees directly with SDCI and King County, and talk to a tax professional about anything affecting your specific tax situation. This guide cannot see your lot, your home’s assessed value, or your finances, and nothing here should be the basis of a financial decision on its own.

Why costs vary so much

Before any numbers, it’s worth understanding why ADU costs span such a wide range, because the “why” tells you more about your likely cost than a single average figure would.

Conversion versus new construction is the single biggest driver. Converting an existing basement or garage into a living space is fundamentally cheaper than building a new detached structure from the ground up, because you’re not paying for a new foundation, a new roof, or new exterior walls. A detached backyard cottage (DADU) needs all of that, plus its own utility connections run out from the house or the street — often the single most expensive line item nobody budgets for correctly the first time.

Site conditions matter more in Seattle than in flatter cities. A sloped lot means more expensive foundation work. Mature or protected trees can force an expensive redesign or relocation of the planned structure. Distance from existing utility lines to the ADU’s location directly drives connection costs. None of this shows up in a generic “cost per square foot” figure, which is why those figures are so unreliable as a planning tool.

Finish level is a real choice, not a fixed cost. A builder-grade finish and a high-end finish on the same footprint can differ by tens of thousands of dollars — flooring, cabinetry, fixtures, and appliances are all genuinely optional upgrades, not fixed requirements.

Size matters, but less than people expect, because fixed costs (permits, utility connections, design fees) don’t scale down proportionally with a smaller unit. A 500-square-foot ADU isn’t necessarily half the cost of a 1,000-square-foot one.

What’s actually fixed and verifiable

These numbers come from primary government sources and can be checked directly. Everything else in this guide is a softer estimate — these are the real, published figures.

SDCI permit fees. Seattle’s Department of Construction and Inspections calculates permit fees from its own “project value” table, not from your contractor’s bid — these are two different numbers, and it’s a common point of confusion. As a concrete, current example: SDCI’s own 2026 fee-change documentation shows a 500-square-foot DADU assessed at a project value of $93,145 carrying a plan review and permit fee of $3,453 in 2026 (up from $2,908 in 2025 — construction and master use permit fees rose 18% year-over-year as part of the city’s 2026 fee update, on top of a 6.5% general inflationary increase to most other fees). SDCI’s 2026 base hourly rate for review work beyond the base fee is $292/hour, and additional surcharges can apply on top of the base permit fee — check SDCI’s current fee schedule for the full stack. This is the building permit fee only — separate trade permits (electrical, plumbing, mechanical) are pulled independently and add several hundred dollars each depending on scope. Confirm current fees directly at SDCI’s fee schedule before budgeting, since these figures change annually.

The King County sewer capacity charge. This is a real, ongoing cost that catches homeowners off guard because it isn’t a one-time permit fee — it’s a separate monthly charge from King County, billed for up to 15 years, that applies whenever a property makes a new sewer connection (which most new ADUs do). The 2026 rate is $77.99 per Residential Customer Equivalent (RCE) per month. King County confirms that new ADU and DADU permits are specifically reported to them for capacity-charge purposes, and SDCI requires an acknowledgment of this charge as part of the ADU permit application. The exact RCE assigned to a given ADU depends on its size and type — confirm the specific figure for your project with King County’s Wastewater Treatment Division directly. You’re allowed to pay the remaining balance in a discounted lump sum at any point during the 15-year term, which is worth asking about if you’d rather not carry a monthly charge.

Property taxes are the genuinely murky one — and worth being honest about. You may see other websites claim ADUs come with a specific multi-year property tax exemption. We looked into this carefully and the picture is considerably less clean than that: Washington does offer a general (not ADU-specific) home-improvement property tax exemption (RCW 84.36.400): it can exempt the value of a physical improvement to a single-family dwelling — up to 30% of the original structure’s value — from property tax for three assessment years, provided the application is submitted before the work is complete. This can apply to ADU construction as a qualifying improvement, but it’s a general program with its own conditions and limits, not a guaranteed ADU benefit. Separately, senior citizens and people with disabilities may be able to include ADU value within existing property tax exemption programs for their primary residence. And realistically, adding a permitted structure to your property will likely increase your home’s assessed value over time, which means somewhat higher ongoing property taxes, not lower ones, outside of any temporary exemption window. This is a genuine “talk to a professional” area — the King County Assessor’s Office can tell you what actually applies to your specific property, and a tax professional can tell you what it means for your return. We’d rather tell you plainly that this is unsettled than repeat a specific-sounding number we can’t stand behind.

Typical total project costs

This section is different in kind from the numbers above — it’s a synthesized range drawn from typical figures reported across the Seattle ADU market, not data aduregister has independently verified. Treat it as an orientation for the conversation with your builder, not a budget.

Attached conversions (basement, garage, or existing-space conversion into a living unit) tend to be the least expensive path, because the structure, foundation, and roof already exist. Costs across the market commonly range from the high five figures into the low-to-mid six figures, depending heavily on how much of the existing space needs new plumbing, electrical, and structural work to become a legal, code-compliant unit.

New detached construction (a backyard cottage built from scratch) is a substantially larger project — new foundation, framing, roofing, siding, and full utility runs. Costs across the market commonly range from the mid six figures upward, with site conditions, size, and finish level all capable of moving the number significantly in either direction.

Two categories consistently surprise people on both project types, worth budgeting explicitly rather than assuming they’re baked into a builder’s headline number: utility connection costs (bringing water, sewer, gas, and power to a detached structure can be a major expense on a difficult site, occasionally rivalling the cost of the structure itself), and the permit and fee stack described above, which for a full DADU project — building permit, trade permits, technology fee, and the ongoing King County sewer charge — realistically adds up to several thousand dollars before construction even starts.

Get itemized bids, not headline numbers. A quoted total that doesn’t break out design, permits, site work, utility connections, structure, and finishes separately makes it very hard to compare bids meaningfully or spot where the real cost differences are.

Financing, briefly

This guide isn’t the place for financial advice, and aduregister isn’t a lender or financial advisor. In broad terms, homeowners typically fund ADU projects through some combination of cash savings, a home equity line of credit (HELOC), a cash-out refinance, or a dedicated construction loan — each with different qualification requirements, costs, and risk profiles. If ADU rental income is part of your plan to help qualify for financing, ask your specific lender directly how they treat that income, since practice varies by lender and loan type. Talk to a mortgage professional or financial advisor about which option, if any, fits your situation.

The one thing that actually gets you a real number

Everything above is context. The only way to get an accurate cost for your project is a site visit and an itemized bid from a licensed contractor who’s actually looked at your lot. This is exactly what aduregister’s builder directory is for: every listed builder is ranked by verified public permit history, not reviews or advertising spend, so you can shortlist experienced local builders before you start collecting quotes. Get bids from more than one.

The disclaimer, one more time — because it matters

This guide gives you a starting orientation, not a budget you can rely on. The fixed fees cited here are dated, published figures that change over time — verify current amounts directly with SDCI and King County before you plan around them. The total-cost ranges are a rough market synthesis, not data aduregister has verified, and your actual cost depends on your lot, your choices, and your builder. Nothing on this page is legal, tax, or financial advice. Before you commit money to an ADU project: get multiple itemized bids from licensed contractors, confirm fees and tax treatment with the relevant agencies, and where the situation warrants it, talk to a tax professional or financial advisor. The cost of good advice is trivial next to the cost of a budget built on the wrong assumptions.

Sources

Last reviewed: July 2026. This guide is general information only — not legal, tax, or financial advice. Fees and tax rules change; verify anything consequential directly with SDCI, King County, and appropriate professionals before acting on it.