Every couple of weeks, we share what we’re seeing across the Puget Sound custom-home market — the trends, costs, and shifts that actually affect a build in Snohomish County and across the greater Puget Sound. This issue is mostly about one question: can a rural lot have a guest house? In Snohomish County the answer starts with a number that is not quite five acres.
A guest house on rural acreage starts with one number: 200,000 square feet
A second, separate dwelling on the same property as the main house is what the county calls a detached accessory dwelling unit, or DADU. It is how you give parents, grown children or long-stay guests a home of their own on your land. In Snohomish County’s rural five-acre zone, R-5, the code prohibits a detached ADU on a lot “less than five acres in size.”
The county does not mean a surveyed five acres. Its code defines “five acres” as 200,000 square feet, or 1/128th of a section when the lot is described that way. Two hundred thousand square feet is about 4.59 acres. An R-5 lot of 4.59 acres or more can have a DADU. A smaller one cannot, however close it looks.
That gap matters when you are shopping for land. A parcel advertised as five acres may measure 4.7 and qualify. One at 4.5 does not. Listing acreage is often rounded, so check the legal description or a survey before you rely on it. Below the threshold you are not shut out entirely: the prohibition applies to detached units, and an ADU attached to the main house is still allowed. Either way, a rural lot gets one ADU, not two.
The county’s other limits are few, and each one shapes the design
The DADU can have up to 1,200 square feet of floor area. Garages, porches, unheated storage and unfinished basements do not count toward that, so a 1,200-square-foot guest house with its own garage is within the limit. It has to use the same driveway as the main house, and the county’s detached ADU bulletin calls for one more off-street parking space.
Then come the two requirements that decide whether the lot can carry a second home at all. You have to document that water is physically and legally available, drinkable, and adequate in flow. And you have to document that the septic system, existing or proposed, can handle the added demand. The county’s own bulletin recommends calling the Health Department before you hire anyone or pay for plans.
Put together, this means the guest house is not something to add to the drawings later. Two dwellings, one driveway, and a septic system sized for both all have to fit the same piece of ground, around whatever the site rules out.
The new state law has not changed the county’s numbers
Washington’s rural ADU law, which took effect June 11, lets counties allow one DADU per rural parcel under a set of conditions: no more than 1,296 square feet, water use metered for each dwelling, and the unit sited within 150 feet of the main house, among others. It is optional for counties, and it does not affect ordinances a county had already adopted.
Snohomish County already allowed rural DADUs, and its code was last amended in March 2025. That code is what is on the books today: 1,200 square feet, not 1,296, and no 150-foot or metering condition listed. If the county updates it, those numbers could move, so confirm them when you are ready to apply. And do not carry them across a county line. Island County and others have their own rules.
Septic inspections become a condition of sale on November 1
Starting November 1, anyone selling a Snohomish County property with a septic system must have a full inspection by a certified contractor within the prior 12 months, pump the tank if the inspection recommends it, and file a Report of Property Transfer with the Health Department before closing. If you are buying acreage with an older house on it, you will now receive that report. If you are selling a home on septic to fund a build, the inspection goes on your list.
Mortgage rates are at a three-year high
Freddie Mac’s 30-year average reached 7.28% on October 1, up from the 6.95% we reported two weeks ago. The Mortgage Bankers Association’s survey for the following week put conforming loans at 7.49% and jumbo loans at 7.39%, the highest in almost three years.
What This Means for Prospective Clients
If a guest house or a home for family is part of what you want from rural property, raise it early, whether you already own the land or are still looking. In Snohomish County’s R-5 zone the lot needs 200,000 square feet, about 4.59 acres, for a detached unit, and the water and septic have to support two homes. Those are facts about the parcel, and no design can change them afterward.
Correction — Our August 26 post said the real limit on rural ADUs is water and septic, not zoning. Zoning is a limit too: in Snohomish County’s R-5 zone a detached ADU is not allowed on a lot under 200,000 square feet. The same post said a detached unit needs its own water meter. That condition is in the state law counties may adopt; it is not listed in Snohomish County’s current code. Corrected October 7, 2026.
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