Some call them Mother-in-Law homes, others a Carriage House, and they have even been referred to as an Accessory Apartment or a Garage Apartment. These are all terms used to refer to extra living space that is private and separated from the rest of the living space in the home. Most often it is in the form of living space housed in a separate building from the main home somewhere within the boundaries of your land/property.

In Snohomish County, these are referred to as an Accessory Dwelling Unit, by permitting and zoning authorities. Sometimes they use the term Guest House. An Accessory Dwelling Unit (ADU) is used when living quarters are for permanent or long-term residents and Guest House is used to describe temporary residents.

In this article, we are going to focus on ADU’s for the purpose of using a shed as a rental property in your yard and what the zoning laws are in Snohomish County.

How Big Can an ADU Be as a Rental?

The laws pertaining to size allowance for an ADU in Snohomish County rely on the size of the main home or primary dwelling unit on the property. Snohomish County states that the floor area of an ADU cannot equal more that 40% of the floor area of the single family dwelling unit (or the main home on the property) it is an accessory to. If the 40% is larger than 850 square feet it is capped off at 850 square feet. No ADU can be larger than 850 square feet.

What Counts as Usable Square Footage?

Going over this the next question that probably pops up is,” What counts as usable square footage?” To determine this, the county uses the same parameters as for normal houses which is net square footage or gross square footage.

Gross Square Footage(GSF): this is calculated from the outside of the exterior walls and is inclusive of the all space within minus any spaces that are open below from the second floor

Net Square Footage (NSF): this is calculated when the square footage of each room is added together. This includes assignable and non-assignable rooms. This does not include wall thickness or space that is open below.

Some Things to Think About

Either measurement method above is valid, but the County is going to want you to use the same method that is used to measure your house.

Stairs and entryways are considered usable square footage

Garages, Porches, and Unfinished Basements are not considered usable space

What Must an ADU Have in Snohomish County?

To be permitted and approved for an ADU the building must:

  • Be on a single family lot (yard) that has only one primary dwelling unit
  • One unit on the property must be occupied by the land owner as their main place of residence (the main house can’t be your vacation home or a rental)
  • The outside design must comply with the City standards in which you live
  • There must be one additional parking space designated for the ADU so the entire property requires at least three spaces
  • There may not be more than three residents living in the ADU who do not qualify as family to the land owner
  • It will need running water/heat/ electricity/etc. if renting
  • It will need to comply with rental codes when renting

What Properties in Snohomish County can Have an ADU?

This is a good question to ask before you start turning a shed into an apartment for renters on your property. The answer to this question depends upon the exact physical address of your home. In some areas ADU’s are not allowed because of the type of neighborhood they are in or in some cases because the HOA laws of a housing community do not allow it.

To determine if your home can legally have a rental ADU on the property, it is best to call the County or the city that you live in and ask them about it, as well as checking in with your HOA if you have one.

Any specific questions you have about building a rental tiny home from a shed on your property can be answered by someone in the City or County you live.

Come and visit us in Marysville today or contact us via phone or email to get started on that perfect private backyard office. We construct Custom GaragesMini CabinsSheds, and Decks

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