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Building a Mother-in-Law in Snohomish County

Updated: Feb 19, 2019

You'll often hear talk of a "Mother in Law Suite", a "Carriage House", or even an "Accessory Apartment". What does that really mean?

#SnohomishCounty #ADU #GuestHouse #MotherInLaw

Let's explore this topic a bit with a focus on Snohomish County in Washington State. We'll call Snohomish County "SnoCo" for short, and use "ADU" for our Carriage House/ Mother-in-Law/ Guest House. This article is specifically for detached ADU's, we'll cover attached ADU's at a later time.

SnoCo has two primary categories for detached ADU's:

  1. Accessory Apartments, and

  2. Guest Houses

Both of which are used to describe a living space, either temporary or permanent in nature, that is built on your property in addition to an existing home (aka your house must be built first to be eligible for these structure types). Each of these has its pros and cons; we think the easiest way to differentiate the two is that ADU's are more regulated, but can be used for revenue-generating rental spaces with permanent residents. Guest Houses, on the other hand, have quite a bit more freedom but are meant to be temporary living spaces.


Let's dive in. Accessory Apartments, (ADU's) are up first.

How big can my ADU be?

The main question we hear when it comes to ADU's is the allowable size. Here's an excerpt from SnoCo Code regarding detached ADU's:

"The floor area of a detached accessory apartment shall not exceed 40 percent of the floor area of the single family dwelling unit to which it is accessory, or 850 square feet, whichever is less."

In other words, you can build an ADU that's 40% as big as your house, so long as it does not exceed 850 total square feet. Keep in mind that the entry way of your Denali or Huntington counts toward that limit.

Looking for a Barn Pros barn with a small apartment? Check out the Caretaker here.

What counts as usable square footage?

Naturally, the next thing you may wonder is what counts for usable size. The County will compare square footage of your ADU to your house using the same measurement (net square footage or gross square footage).

Check out this PDF from the University of Cincinnati for definitions, but here's a quote to summarize:

Gross Square Footage (GSF): The Gross Square Footage is calculated from the outside of the exterior walls and is inclusive of all space within minus areas that are open to below.
Net Square Footage (NSF): The Net Square Footage is the total square footage of all the rooms/areas on a floor. This includes assignable and non-assignable rooms. Note: NSF calculations do not include wall thickness or space that is open to below.

Essentially, you can measure square footage with or without wall thickness to your liking, but you must use the same method when measuring your house for SnoCo to play ball.

A few considerations:

  • Stairs and entryways are considered Usable Space... design wisely!

  • Garages, Porches, and Unfinished Basements are not considered usable space and are not counted with the total. Hooray!! (Barns also fall under the exemption).

If the design you're considering is beyond the square footage limit, you may consider finding a model that fits or opting for a Guest House instead.

Other Considerations

We recommend checking out SnoCo's code on Accessory Apartments for more information. Here's a link. There are a few other items to consider, but they're fairly self-explanatory. For example, your ADU must have an approved Septic System, at least one parking space, and must look somewhat similar to your house. (Usually paint, roofing type / color, and / or accents such as windows and light fixtures will do the trick).

We'll have a future article specifically about #Septic Systems and Health Department Approvals, so stay tuned for more.


Guest Houses carry many similarities, but here's a definition from SnoCo:

"'Guest house' means a structure with not more than two bedrooms having no kitchen, and which shall be used or designed for use primarily by guests or servants for sleeping quarters only."

No Kitchen??

You read that right. No Kitchen! However, the definition of a Kitchen in SnoCo's eyes allows for a bit of wiggle room.

"Kitchen" means any room or area used, intended, or designed to be used for the cooking or preparation of food which contains any two of the following: a kitchen type sink, refrigerator, range, or 220 H2 outlet.

What you do or don't put in your Guest House is between you and the county, but by the code it seems you can pick what you'd like to forego in the spirit of size.


Permitting Considerations

ADU's require a special type of Land Use Permit, called an Administrative Conditional Use Permit (ACUP) to be allowed on your property. This comes complete with a newspaper posting, a 14 day appeals period, and one of those big orange NOTICE OF LAND USE signs out front of your house for the duration of the permit.

Guest Houses, on the other hand, need none of that! This ends up being a very real savings of time and at least a few thousand dollars of engineering and permitting expenses.

When it comes down to the brass tax, Guest Houses are easier to permit and have much more flexibility than ADU's. Given their intent of non-permanent dwelling, they're not perfect for every property owner, but, the choice is yours.



Permitting can be an overwhelming task, but we're here to help. Your build by BPC includes the BPC Permit BOOST Program, which has been designed to help make your life easier as you navigate the wonders of your local building department. Better yet, can opt for our Turn-Key Permit Service if you'd like to save yourself dozens of hours and lots of headaches while getting your permit faster.

At the end of the day, the best resource when it comes to what you can build is your local building department. They exist to keep you safe and to make the community a nicer place, so we recommend dropping by their offices for more information.

Snohomish County PDS

3000 Rockefeller Ave

Everett, WA 98201

(Second Floor)

PS. There's a great cafe/ lunch spot on your way out!

-Jake K.


For more information on Accessory Apartments, visit the Snohomish County Code here:

Guest Houses are here:

Definition of a Kitchen:



University of Cincinnati

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