In Seattle, ADU rules have recently been updated. Learn about the regulations and requirements you need to follow now when building an ADU in Seattle.
Updated May 30, 2023
ADU Regulations for Building an AADU and DADU in Seattle
ADUs are a great solution to increasing the availability of affordable alternative housing, as they offer private living at little to no additional land cost and can support different types of housing needs.
Seattle’s ADU Reforms
Attached ADUs (AADUs) have been around in Seattle since 1994. In 2019, however, some building regulations were reformed that made detached ADUs (DADUs) more accessible and popular over the past few years.
Seattle City Council reforms passed in 2019 allow more than one ADU per lot in some cases, allow ADU construction on smaller lots, and don’t tie ADU eligibility to owner-occupancy.
There are some requirements that are still applicable across the board; however, depending on whether your lot is zoned for single-family or multi-family use, you may have to meet additional regulations when building your ADU. There are three main types of zones in Seattle as below:
NR1, NR2, and NR3 - Neighborhood Residential Zones
Previously known as SF 9600, SF 7200, and SF 5000 respectively, NR1, NR2, and NR3 are Single Family Zone codes based on minimum lot area:
NR1 - 9,600 sq. ft.
NR2 - 7,200 sq. ft.
NR3 - 5,000 sq. ft.
RSL - Residential Small Lot
These areas are suited for the development of smaller homes and are typically more affordable than NR Zones.
LR1, LR2, and LR3 - Low Rise Zones
LR Zones are zoned for multi-family residences such as row houses, townhouses, and apartments.
Curious about the rules for your zone? Jump ahead:
General Requirements for Building an AADU or DADU in Seattle
The following are some of the basic aspects of Seattle’s building regulations that are applicable to most zones:
Minimum lot size, width, and depth
In Seattle, your lot must be at least 3,200 sq. ft. to build a new ADU, but you may be able to build a conversion ADU on a smaller lot, depending on the exact location of your lot.
Your lot should also be at least 25 ft. wide and 70 ft. deep to accommodate a DADU.
Unless all residents are related to each other, there may be no more than eight residents between an ADU and the primary residence.
If building an ADU on your lot would require you to cut down any trees, your ADU may not occupy more than 40% of your lot area. Otherwise, your ADU may occupy up to 60% of your lot area.
AADUs may follow the existing setback requirements that apply to the primary residence, but DADUs have a few specific requirements.
DADUs may not be built in front yards, so they only need to adhere to side and rear setback rules. Typically, you need to maintain a 5 ft. distance between your DADU and lot lines or other structures. DADUs on corner lots should maintain lot line setbacks of 10 ft. and if your lot borders a back alley, you may build your DADU right up to it.
As of 2019, ADUs aren’t required to have their own off-street parking in any zone. Homeowners may create a new parking space for the occupant of the ADU, but this is not a requirement.
Replacement parking is required if a parking space belonging to the primary residence is lost or removed to accommodate an ADU.
The Rules For Building an ADU in Seattle’s NR Zones
NR Zones are zoned for single-family living and the following requirements are applicable to most single-family homes:
Number of ADUs
Lots in these zones may have up to two ADUs, only one of which can be a DADU. So, on a single-family lot, you may build either:
Two attached ADUs (AADUs),
One DADU, or
One AADU and one DADU.
Both DADUs and AADUs in Neighborhood Residential Zones should be under 1,000 sq. ft. For AADUs, garages don’t count toward this maximum area.
The following areas don’t count toward the maximum area for DADUs:
Porches and covered decks under 25 sq. ft.
Bike parking areas under 35 sq. ft.
This floor area may either be in one story, or distributed across multiple stories.
Height limits on your structure would depend on the width of your lot. This allowance is then divided into a base height limit and additional height that depends on the roof type.
Here’s a quick breakdown of these limits, based on lot width:
If your lot is less than 30 ft. wide, you are allowed a base height of 14 ft. and an additional 3 ft. for either a pitched roof, shed roof, or butterfly roof
If your lot is between 30 ft. and 40 ft. wide, you are allowed a base height of 16 ft., an additional 7 ft. for a pitched roof, and 4 ft. for a shed roof or butterfly roof
If your lot is between 40 ft. and 50 ft. wide, you are allowed a base height of 18 ft., an additional 5 ft. for a pitched roof, and 4 ft. for a shed roof or butterfly roof
If your lot is wider than 50 ft., you are allowed a base height of 18 ft., an additional 7 ft. for a pitched roof, and 4 ft. for a shed roof or butterfly roof
If your structure has a green roof or another need to meet a green building standard, it may exceed the maximum allowed height by 2 ft.
Lot coverage and floor area ratio (FAR)
Lot coverage refers to the percentage of your total lot area occupied by structures. For lots 5,000 sq. ft. or larger, a maximum lot coverage of 35% is allowed. Decks built under 36 inches above ground level and eaves that project less than 36 inches from your DADU or house do not count towards your lot coverage.
FAR is the area on your property occupied by structures divided by the total lot area, expressed as a ratio. In Seattle’s Neighborhood Residential Zones, FAR is limited to 0.5, and the floor area of ADUs does not contribute to your lot’s FAR limit.
Since AADUs are contained entirely within an existing primary residence, they do not have specific yard requirements. The following rules apply to DADUs:
DADUs should be outside the required front yard and side yards
Front yards should span 20 ft. between your main house and the street, or the average distance of the front yards belonging to the single-family lots on either side of your house, whichever is less
Unless your side lot line continues from a neighbor’s front lot line or abuts an alley, side yards must be 5 ft. from your primary dwelling
Rear yards must be at least 10 ft., and a maximum of 25 ft. from your primary dwelling or 20% of lot depth, whichever is less
DADUs and other accessory structures may occupy a maximum of 60% of your rear yard
If you maintain a 10 ft. easement that separates your DADU from your neighbor’s structure(s), your DADU may extend into a side yard
You may not build a DADU in the rear yard of a reversed corner lot if that portion of your rear yard abuts your neighbor’s front yard
If you have an existing accessory structure in a side yard, you may convert it into a DADU
As long as they are not within 3 ft. of a lot line, eaves, bay windows, and certain structures may project slightly into a yard
The Rules For Building an ADU in Seattle’s RSL Zones
The requirements for maximum size, maximum height, and minimum lot size, width, and depth are the same for NR and RSL Zones. The following criteria differ for RSL-zoned lots:
Number of ADUs
RSL-zoned lots may have one ADU, either attached or detached, associated with each primary dwelling on a lot.
For example, if a lot has five primary dwellings/homes on it, then each home is allowed to have one ADU.
The total square footage allowed for a dwelling unit in the RSL Zone is 2,200 sq. ft., and this allowance includes the area of an ADU.
The following limits to new ADUs (added to existing primary dwellings after April 2019) exist:
The ADU should add less than 20% of the existing floor area to the property
The ADU may only add floor area by adding or expanding a second story directly above an existing portion of the primary dwelling
Underground areas and portions of a story that are more than 4 ft. above ground don’t count towards the size allowance.
Lot coverage and floor area ratio (FAR)
All RSL-zoned lots are allowed a maximum lot coverage of 50%. If your lot abuts an alley, half of the width of the alley is included in this calculation.
In RSL Zones, FAR is limited to 0.75, and the floor area of ADUs contributes to your lot’s FAR limit.
RSL Zones have fewer yard requirements for DADUs than NR Zones:
DADUs should be outside the required front yard and side yards
Front yards should span 10 ft. between your main house and the street
Rear yards should be 10 ft., but rear yards aren’t required if the rear lot line abuts an alley
Side yards should be 5 ft., but side yards aren’t required if the side lot line abuts an alley
The Rules For Building an ADU in Seattle’s LR Zones
Number of ADUs
Lots in LR Zones may have one ADU, either attached or detached, associated with each primary dwelling on a lot. This applies to all single-family, rowhouse, and townhouse units in Seattle’s LR Zones.
Both DADUs and AADUs in LR Zones should be under 650 sq. ft.
ADUs also may not exceed 40% of the lot’s total residential floor area.
DADUs maybe 20 ft. in height but are allowed an extra 3 ft. for a pitched roof. Shed roofs and butterfly roofs must be contained within the 20 ft. height allowance.
ADUs must be located either within the primary dwelling (for AADUs and conversion ADUs), or between the primary dwelling and the rear lot line (for DADUs).
Permitting an ADU in Seattle
Before you begin your project, you need to get a building permit approved by the city planning department. This part of the process should take between four and six months, but timelines can vary based on jurisdiction.
Other permits, like electrical permits and plumbing permits, may be required on a case-by-case basis, such as if you need to connect to an existing side sewer. Construction and alteration permits are required for adding to existing units (AADUs), legalizing existing units, and building DADUs.
The admin side of your project can be time-consuming, but getting the right permits is arguably the most important part. Cutting corners can lead to extra costs in the future, so reach out to an expert if you need help navigating the red tape.
ADU Fees in Seattle
Fees fall under soft costs, and can often amount to around a tenth of your total project costs. Fees can depend on a number of factors, and sewer, electrical, and construction fees vary based on square footage and other project specifics. For example, a DADU can incur about $30,000 in total soft costs out of a total budget of $300,000.
Working with an experienced team of ADU specialists can give you access to reliable contractors and discounted bulk pricing, reducing the costs associated with building your ADU.
How Can Cottage Help You Build an ADU in Seattle?
ADU construction is booming in Seattle, with homeowners enjoying the benefits they bring in terms of property value and affordable housing. Our team of experts has worked on a variety of ADU projects and helped homeowners navigate eligibility, design, permitting, and construction from start to finish, and we can do the same for you.
Reach out to schedule a no-obligation call with one of our experts, and get started on building your fully customizable ADU.
What are the rules for AADUs in Seattle?
In Seattle’s NR and RSL Zones, AADUs under 1,000 sq. ft. are allowed. They may be 650 sq. ft. in LR Zones. Across the board, however, AADUs must meet the current standards for residential, building, mechanical, electrical, energy, land use, environmentally critical areas, and shoreline codes.
Can I build an ADU on my property in Washington State?
Yes, you may build ADUs in single-family and low-rise zones and even neighborhood or commercial zones in some cases. However, as with most other areas, you will need to obtain a few permits before you are able to begin constructing your ADU.
How Much Does it Cost to Build an ADU in Seattle?
The typical price range for a custom stick-built ADU in Seattle is $200,000-$300,000, but this figure depends on a number of factors: type, size, lot-specific characteristics, labor costs, finishes, and other factors associated with the scope of your project.
Are tiny houses legal in Seattle?
Yes, assuming they are built in accordance with the Washington State Building Code, you can build a tiny house in Seattle. The approval process does depend on jurisdiction though.
Does an ADU add value in Seattle?
Yes, as with other useful amenities, structures, and features, the inclusion of an ADU can increase your property value as you are adding living space.
Can a DADU be sold separately in Seattle?
This would depend on a number of factors. Typically, most lots are too small to be subdivided under Seattle's land-use code. As a result, DADUs usually can’t be sold as separate units. However, the primary residence and its DADU may be sold separately if they are on a lot owned by a condo association.
Are more ADUs being built in Seattle?
Yes, the city has seen a big jump in the number of permits submitted for ADUs between 2019 and 2022 – roughly a 250% increase.
What are the benefits of building a DADU in Seattle?
DADUs can bring in more rental value and a higher ROI than AADUs and other ADU types. This is because they offer rental flexibility, have the potential to be used in many different ways, and offer privacy and independence owing to their separation from the primary residence.
Want to learn more about ADU regulations in your area?
Book a free consultation and estimate with a Cottage ADU Expert to find out what’s possible on your property!