What is the cheapest way to add an ADU in Seattle?

Converting space that already has a roof over it. A basement unit runs $120,000 to $180,000 and a garage conversion $150,000 to $200,000 because the foundation, the roof and the utility connections are already there. A detached cottage starts around $250,000 for the simple reason that it needs all three built from scratch.

Does the per-square-foot price include finishes?

No. Our $350 to $450 per square foot covers design, engineering, the permit and the construction itself: excavation, foundation, framing, roof, siding, plumbing, electrical, insulation and drywall. Tile, flooring, cabinets, fixtures and appliances are picked by the homeowner and billed at cost. That is why two cottages of the same size can end up $30,000 apart.

How long does an ADU take in Seattle?

For a detached unit inside the city, count on four to five months for the SDCI permit and six to eight months of construction, so roughly ten to fourteen months from the first site visit to keys. Attached units and conversions are quicker on both ends, usually eight to twelve months in total.

Can I rent out an ADU in Seattle?

Yes, and you do not have to live on the property to do it. Seattle dropped the owner-occupancy rule in 2019. Attached units in our projects rent for $1,600 to $2,600 a month, detached cottages for $2,000 to $3,500, depending on size and neighborhood.

What an ADU Costs in Seattle in 2026

In short

In 2026 a Seattle homeowner should expect to pay $120,000 to $180,000 to turn a basement into an ADU, $150,000 to $220,000 for a garage conversion or an attached unit, and $250,000 to $400,000 for a detached backyard cottage of 600 to 1,000 sq ft. Construction runs $350 to $450 per square foot. Finish materials are on top of that.

Cost by type of unit

The single biggest decision is not the size or the finishes. It is whether the unit shares a wall with your house. An attached unitborrows the house’s foundation, roof and utilities. A detached one needs all of its own. Everything else in the budget follows from that.

Type	Usual size	2026 range	Where the money goes
Basement ADU	500–900 sq ft	$120K–$180K	Egress window, plumbing under the slab, ceiling height, a second kitchen.
Garage conversion	400–600 sq ft	$150K–$200K	Insulating the slab, closing the door opening, new plumbing and electrical.
Attached addition	500–1,000 sq ft	$160K–$220K	New foundation and roof, but tied into the house and its utilities.
Detached cottage (DADU)	600–1,000 sq ft	$250K–$400K	A complete small house. Own foundation, roof, sewer and power run.
Two-story or premium DADU	800–1,000 sq ft	$400K–$600K	A stair, more structure, high-end finishes, or a lot that fights you.

One thing that surprises people: the range for a garage conversion overlaps with an attached addition. A garage slab was poured to hold a car, not to be lived on. It has to be insulated, often cut for plumbing, and the big door opening has to be framed and finished as a wall. By the time that is done you have spent a good part of what a new addition would have cost.

What the price covers, and what it does not

At $350 to $450 per square foot, the construction number typically carries the whole job through final inspection. Design and drawings. Stamped engineering. The SDCI permit and every inspection along the way. Excavation, foundation, framing, roof, siding, windows and doors, plumbing, electrical, heating, insulation, drywall and paint-ready walls. The price is fixed once the engineering is done, before we submit for the permit, and it does not move unless you change the scope.

What is not in it: finish materials. Tile, flooring, cabinets, countertops, faucets, light fixtures, appliances. You choose them, we install them, and you pay what they cost. We do it this way on purpose. A $200 faucet and a $900 faucet take the same hour to install, and you should be the one deciding which hour you are paying for.

Kitchen inside the View Ridge attached ADU: navy cabinets, quartz counters, skylight — Inside the View Ridge unit. Cabinets, quartz and the skylight were the homeowner's choices, billed at cost on top of the construction price.

The extras that only a detached unit has

If you are comparing a cottage against an attached unit, these three lines explain most of the gap.

Utility connections, $18,000 to $35,000. Water, sewer and power have to reach the back of the lot. How far the sewer main is, and whether the new line can drain by gravity or needs a pump, decides where in that range you land.
Site work, $35,000 to $110,000. Excavation, foundation, drainage, grading. A flat lot with decent soil is the low end. A slope, a high water table or a tree we have to protect is the high end.
Trees. Seattle protects exceptional trees and their root zones. If one sits near the footprint, expect an arborist report and possibly a redesign before the permit moves.

Where the city helps:ADUs are exempt from parking requirements, from street-improvement requirements and from Seattle’s MHA affordable-housing payment. The one charge that does follow a new unit is the King County sewer capacity charge, billed monthly for 15 years after the sewer connection. On a conversion or an attached unit the extras above mostly do not apply at all.

Aerial view of the View Ridge lot: the attached ADU sits behind the main house, the lawn and trees untouched — View Ridge from above. Attaching the unit to the house kept the sewer run short and left the yard and the trees where they were.

Seven finished projects, with the numbers

Every price below is the contract price, not an estimate. Each card opens the project page with photos and the scope. The full list is on the ADU and DADU projects page.

Gray single-story backyard cottage with a covered entry in Bothell, built by Evercrest — Bothell, 680 sq ft, $285,000. Built for a tenant from day one, so the finishes are durable rather than fancy.

Look at the Bothell pair. The 550 sq ft cottage and the 680 sq ft one are $65,000 apart, and only a small part of that is the extra 130 sq ft. The rest is the deck, the site and the finish choices. A foundation, a sewer line, a kitchen and a bathroom cost about the same whether the unit is 500 or 900 sq ft, which is why a slightly bigger unit is almost always the better deal per square foot.

What moves the number up or down

Down

Converting space you already own instead of building new.
A flat lot with the sewer line close to where the unit will sit.
A simple rectangle with a gable or single-pitch roof.
One bathroom, a compact kitchen, mid-range finishes.
An alley behind the lot: zero rear setback, and deliveries do not cross your yard.

Up

A slope, soft soil or a high water table under the foundation.
A second story. More structure, a stair, and more roof and siding per square foot of floor.
A protected tree inside or next to the footprint.
A long utility run, or a sewer line that needs a grinder pump.
Custom cabinetry, oversized windows, imported tile.

Permits and timeline

Inside the City of Seattle, a detached ADU permit through SDCI takes about four to five months for a custom design; the city itself warns that the first intake appointment alone can be two to four months out. With one of the city’s pre-approved DADU plans SDCI quotes two to six weeks for the permit, at the price of building a stock design. Building a typical cottage then takes six to eight months. Attached units and conversions are faster on both ends. In unincorporated King County the permit is usually closer to two months.

The View Ridge project above went from an approved plan set to keys in ten months, six of them construction. We drew it, engineered it and walked it through SDCI ourselves, which is the main reason it did not sit in a queue between an architect and a builder. That is what design-build under one contract means in practice.

Does it pay back?

In most Seattle neighborhoods, yes, and in cash rather than only on paper. There is no owner-occupancy rule, so both the main house and the unit can be rented. The numbers our clients see:

Attached unit or conversion: $1,600 to $2,600 a month in rent.
Detached cottage: $2,000 to $3,500 a month.
Property value: roughly $200,000 to $350,000 added for a detached unit.
Break-even on a rented unit: about seven years at current rents.

And since Washington’s HB 1337 took effect, a detached unit can be sold on its own as a condominium unit; the lot itself stays whole, Seattle does not allow splitting an ADU off by subdivision. For anyone thinking about retirement that changes the math again.

Still deciding between attached and detached? We wrote up how to choose in ADU or DADU? How to choose on a Seattle lot.