A detached ADU on the Eastside: $325,000 to $550,000, all in
Published 2026 ranges for the Seattle and Eastside market, for a finished, permitted, connected building — not the shell.
| Size | All-in total | Per square foot |
|---|---|---|
| 450–600 sq ft studio or one bedroom | $325,000–$400,000 | $540–$720 |
| 800 sq ft one or two bedroom | $350,000–$450,000 | $440–$560 |
| 1,000 sq ft two bedroom | $375,000–$550,000 | $375–$550 |
| Garage conversion (existing structure) | $90,000–$190,000 | — |
The counter-intuitive part: the small one costs more per square foot, not less. A kitchen, a bathroom, a panel, a water heater and a sewer connection cost the same whether they sit in 450 square feet or 1,000. Below about 600 square feet you are paying full price for the expensive half of a house and getting very little cheap floor with it.
Eastside lots — Bellevue, Kirkland, Redmond, Mercer Island — sit at the upper end of these ranges rather than the lower one. Steep lots, mature trees and long utility runs are the three things that move a project from the middle of the range to the top of it.
These are published market ranges for 2026, not our quote. Our number for your lot is written and itemized, and it comes after the walkthrough — never over the phone.
Why the ad says $295,000 and the invoice says $430,000
The advertised number is almost always the building. Here is what sits underneath it on a real lot.
| Line item | Typical 2026 cost | In the "from" price? |
|---|---|---|
| Construction of the building itself | $400–$650 per sq ft | Yes — this is the number you saw |
| Architecture, engineering, site plan | $12,000–$25,000 custom; $1,000 licence fee on a Seattle pre-approved plan | Rarely |
| Foundation and site preparation | $30,000–$50,000; more on a steep lot | No |
| Utility connections — water, sewer, power | $25,000–$60,000 | No |
| King County sewer capacity charge | about $14,000 | No |
| City permits and trade fees | $6,000–$14,000 in Seattle; $20,000–$40,000 all-in permits and fees is common | No |
| Tree work, drainage, retaining, driveway | Site by site | No |
Add those up on an ordinary lot and you are between $120,000 and $180,000 above the advertised building price before anyone has picked a faucet. That gap is not a scam and it is not a surprise to the builder — it is simply what a second dwelling costs once it has to connect to the world.
What we do about it: our written estimate carries every one of those lines with a number next to it, before you sign anything. If a line cannot be priced until the ground is open — a utility run of unknown depth, for instance — it is called out as an allowance with a range, not buried.
The rules changed, and they are still changing
Washington rewrote the ADU rules statewide, and the Eastside cities are still catching up. What was impossible on your lot three years ago may be routine now.
The statewide floor
Under Washington's 2023 ADU law, a city over 25,000 people must allow at least two ADUs on a residential lot — two detached, two attached, or one of each — and cannot cap ADU floor area below 1,000 square feet. No city in Washington may require the owner to live on the property, and no city may require extra off-street parking for an ADU within half a mile of a major transit stop. Large cities were to comply by June 2025, smaller ones by December 2025.
Bellevue
Up to two ADUs per lot. Detached units up to 1,200 square feet, 24 feet tall — 28 feet over an existing accessory structure. No parking required under 1,000 square feet. No owner-occupancy. Approval is administrative and by right, with no public hearing. Bellevue also permits a DADU to be sold separately, which changes the investment arithmetic completely.
Kirkland
Detached and attached ADUs in the RSA and RSX zones, up to 1,200 square feet and 24 feet, with a 10-foot rear setback. Median permit approval in 2026 runs eight to sixteen weeks from a clean submittal — the words "clean submittal" carrying most of the weight in that sentence.
Seattle
Seattle's January 2026 zoning overhaul raised the height limit from 14–18 feet to as much as 32, took lot coverage to 50 percent, and allows two DADUs per lot. The city's pre-approved plan catalogue cuts design fees to a $1,000 licence and permit review to two to six weeks instead of several months — but the current ten designs were drawn for the old envelope, and a refreshed catalogue is due in November 2026. If you are building in Seattle in 2027, that timing is worth a conversation.
Codes in this region are being rewritten faster than any page can keep up with. We verify your parcel against your city's current code — not against a summary — as the first step of the feasibility check.
What a finished unit rents for
Published 2026 figures for the Seattle and Eastside market put a one-bedroom ADU at roughly $2,400–$2,900 a month and a two-bedroom at $3,100–$3,900, with an estimated return in the range of six to eight percent a year on the build cost, before the effect on the value of the property itself.
We are builders, not financial advisers, and we will not tell you what your unit will rent for. What we will do is give you a real construction number early enough that you can take it to someone who does that for a living, and get an answer before you have spent anything but an afternoon.
The free feasibility check
Before design, before drawings, before money. We answer within two business days.
Your zone and your envelope
What your city's current code allows on your parcel: how many units, how big, how tall, and where they can sit after setbacks and lot coverage.
Where the utilities are
Sewer, water and power — where they run, how far they have to come, and whether the capacity charge applies. This is the line item that ruins budgets.
What is in the way
Trees, slope, drainage, easements, critical areas. The things that turn an eight-week permit into a nine-month one.
A straight answer
What you can actually build, roughly what it will cost, and how long it will take. If a DADU does not pencil on your lot, we tell you that, and it has cost you nothing to find out.
Frequently asked questions
Why do the ads say $295,000 when you say $400,000?
Because the advertised number is usually the building alone — the box on the slab. The invoice also carries site work, the foundation, the utility runs from the street or the house, the King County sewer capacity charge, permit and trade fees, and the design. Those are real, they are not optional, and on a typical Eastside lot they add well over a hundred thousand dollars. Nobody is lying in the ad; the ad is simply not the whole number.
What is the cheapest honest way to add a rental unit?
Converting space you already have. A garage conversion or a basement unit reuses a foundation, a roof and, usually, a shorter utility run. Published 2026 ranges for the Eastside put a garage conversion near $90,000–$190,000 against $300,000+ for a new detached building. If the goal is rent rather than a specific building, we will say so at the walkthrough.
How long does a DADU take?
On the Eastside, plan on nine to fourteen months from the first call to the final inspection. Design and engineering take two to four months, the city takes eight to sixteen weeks from a clean submittal, and construction runs five to seven months. Tree removal, a critical area or a steep lot adds to the front half, not the back half.
Can I rent it out? Do I have to live on the property?
You can rent it, and you do not have to live there. Washington preempted owner-occupancy requirements statewide under RCW 36.70A.681, so no Eastside city can require you to live on the lot as a condition of having an ADU.
How many can I build?
Under Washington's 2023 ADU law, cities over 25,000 people must allow at least two ADUs on a residential lot — two detached, two attached, or one of each — and cannot cap the floor area below 1,000 square feet. Bellevue allows up to two and up to 1,200 square feet detached. Kirkland's code as written allows one. We confirm your lot against your city's current code in the feasibility check, because these codes are still being rewritten.
Do I need a parking space for it?
Usually not. Cities cannot require additional off-street parking for an ADU within half a mile of a major transit stop, which covers most of the Eastside's established neighborhoods. Bellevue additionally requires none under 1,000 square feet.
What does the feasibility check cost?
Nothing. We look at your parcel, your zone, your setbacks and lot coverage, where the sewer and the water are, and whether there is anything on the lot that will stop you — and we tell you what you can actually build there. If the answer is that a DADU does not pencil on your lot, we say that, and it costs you nothing to have found out.
Where these numbers come from
Cost, rent and code figures on this page are drawn from published 2026 Seattle-area sources, current as of September 2026: cost ranges and per-square-foot figures from Ballard Backyard Cottages and Emerald City Construction; the soft-cost and pre-approved-plan breakdown from MB Remodels; Bellevue and Kirkland code summaries from ADU Zoning and Golden ADU; statewide requirements from Washington's 2023 ADU law and RCW 36.70A.681. They are market ranges for the region, not a quote for your project.
Pick your city. The ADU rules are different in every one of them.
Washington rewrote the accessory dwelling unit law statewide and each city rewrote its code to match — on its own schedule, with its own numbers. We read all thirteen codes and wrote down what each one actually allows in 2026: how many units, how big, how tall, what parking, and whether you can sell it.
What it costs, everywhere: a new detached unit runs roughly $325,000 to $550,000 all in on the Eastside, and the advertised "starting at" prices leave out $120,000 to $180,000 of site, utility and permit work. The whole number, line by line →
Ready when you are
A free walkthrough, a written itemized estimate, and no obligation. Not sure yet? Send us photos of the lot first.