What a backyard home really costs in Snohomish County

The honest answer to “what does a backyard home cost” is a range, and the range is wide, and the reasons it is wide are more useful than the number. Here is the number anyway, and then the reasons.

The headline range

For a detached backyard home in Snohomish County, the published range from an Everett remodeler’s 2026 cost guide is $270,000 to $450,000 and up, or about $340 to $500 per square foot all in. The same guide puts an attached unit at $150,000 to $300,000 and a garage conversion at $80,000 to $180,000. Regionally, builders quote $250,000 to $650,000 for detached units, with Everett and Tacoma among the more affordable markets and Bellevue and Issaquah higher because of sloped lots and design review.

A real, recent, permitted detached unit built by HousUp's partner builder in Washington lands inside that band: a 998-square-foot unit on Beacon Hill at $389,333.

The partner builder's 2026 package prices are $220,737 for the compact plan (the Alder), $291,458 for the Willowmere two-story (its roof deck and garage are add-ons), and $302,413 for the Backyard Residence. Each covers the vertical build, foundation, excavation and grading on a flat lot (carried at $35,000, in a $30,000 to $40,000 range), and design and engineering. Permit fees, utility connections, sales tax and any site work beyond a flat lot are separate lines, which is why a package price sits toward the low end of the all-in band or below it. Those are working figures, not a price list; the report gives you a range for your parcel, and a fixed price only comes after design and a site visit.

Why small units cost more per foot

Every builder’s table shows the same thing: a 450-square-foot studio can cost more per square foot than a 1,000-square-foot two-bedroom. The kitchen, the bathroom, the utility connections and the foundation cost about the same whether they serve 450 feet or 1,000. That is why the report’s size band matters so much: a lot that can take a two-bedroom unit often gets a better project for the money than one limited to a studio.

The line items that move the number

These are the hard-cost drivers as published by Puget Sound builders in 2025 and 2026. Your lot will hit some of them and not others; that is most of the difference between $270,000 and $450,000.

Driver	Published range	What decides it
Site prep and foundation	$30,000–$50,000	Slope, soil, access for equipment
Utility runs (water, sewer, power)	$30,000–$50,000; wider bands of $25,000 to $100,000+ for utilities plus site work	Distance from the main house to the connection points
Sewer connection	$5,000–$15,000	Whether a lateral exists and where it runs
Water connection	$3,000–$8,000	Meter size and distance
Electrical service	$2,000–$6,000 in the city; $3,000–$12,000 rural	Panel capacity, trenching
Septic (unsewered lots)	$15,000–$35,000	Room for a replacement drainfield
Energy-code compliance	$5,000–$15,000 (Everett) to $30,000–$70,000 (Seattle builders)	The 2021 code’s envelope and heat-pump requirements
Plumbing, electrical, HVAC	$40,000–$60,000 (15–20% of the build)	Fixture count, heat-pump water heater
Geotechnical report	$3,000–$7,000 when required	Slope, fill, or the city asks
Contingency	10–20%	Age of the lot’s records, what the excavation finds

Soft costs: design, engineering, permits

Before a shovel moves, there is design, structural engineering, a survey, utility investigation and the permit package. Builders publish these as $20,000 to $50,000 combined in Seattle and $35,000 to $80,000 in rural King County, or as 10 to 15 percent of the whole project. In Everett, permits, plan review and structural engineering typically run $3,000 to $8,000, and Everett halves park impact fees for the first two ADUs and waives transportation impact fees on the first one entirely. State law caps ADU impact fees everywhere at half of what the main house would pay.

In the partner builder's 2026 proposals, design and engineering is carried as a $29,000 line inside the price, not a fee on top, and it covers survey, design, engineering, utility investigation and the full permit package, ending in a fixed-price proposal you can accept or decline.

What is not in the number

Furniture, landscaping, and a driveway or parking pad if your city or your own use calls for one. Fire sprinklers, which depend on the distance from the street and are not reliably priced by anyone; treat them as a site-specific question. And financing costs, which depend entirely on how you pay.

How people pay for it

Cash is more common on smaller units than people assume. A home equity line of credit sits behind your first mortgage and leaves your existing rate alone. A construction or renovation loan funds in draws and needs plans and a signed contract, so it comes after the design package. A cash-out refinance replaces your mortgage, which means it replaces your rate; if you hold one under 4 percent, that is usually the thing you were trying to protect. We do not lend or broker loans.

What the report does with all this

It doesn’t quote you a price. It tells you which size band may fit on your parcel, which of the drivers above your lot is likely to hit (sewer versus septic, slope, access, trees), and what comparable projects have cost recently, with every rate sourced and dated. A fixed price comes after design and a site visit, from the builder of record named in your proposal, and only if you ask for it.

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