ADU Requirements & Permits: Exploring Seattle's Housing Options
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ADU Requirements & Permits: Exploring Seattle's Housing Options

|Aug 23, 2023

Seattle, a city renowned for its vibrant urban landscape and green spaces, is witnessing a transformation in housing opportunities. ADUs, more popularly known as secondary units or backyard cottages, are becoming an increasingly popular choice among residents. The growing interest in sustainable living has also led to a surge in the popularity of prefab homes in Seattle.

But what does ADU stand for? What are the specific requirements of ADU Seattle, and how can you navigate the landscape of prefab and tiny homes in this buzzing city? We have all the answers for you, so let's dive in.

What Is an ADU?

ADU stands for Accessory Dwelling Unit, a secondary housing unit that shares the same lot as a primary dwelling. They are typically smaller than the primary dwelling and can be built in various ways depending on local building regulations. Primarily, every ADU serves the central role of providing a self-contained living space equipped with all necessary amenities for independent living.

To classify a unit as an ADU, it must have all facilities required for sleeping, dining, cooking, and sanitation. There are generally two types of ADU structures:

Attached ADUs in Seattle

This type of ADU is built as an extension, or an addition, to a primary building. It shares plumbing and electricity supply with the main building.

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Detached ADUs in Seattle

This is a separate structure, often located in the backyard of the primary home. It's independent and may look like a tiny home or a cottage. They typically have a separate foundation, plumbing, and electricity or gas supply system.

ADUs provide a unique opportunity for homeowners to leverage their properties for additional living space or rental income. They can also be used to house extended family members or elderly parents. These structures have gained popularity in many cities, increasing housing availability and affordability. However, regulations for building and using ADUs vary by city and state, so it's important to check local laws and guidelines. In Seattle, ADU regulations and rules differ for attached or detached versions. So let's explore that next.

What Is an ADU?

Rules and Regulations of ADUs in Seattle

For a tiny home, Seattle is the best place since the local rules are pretty relaxed in comparison. Since ADUs are like mini-houses in your backyard, there are some rules set out by the local authorities, in Seattle's case, the Seattle Department of Construction and Inspections (SDCI), that one needs to abide by for their ADU to be considered legal. You can build these in residential neighborhoods, low-rise zones, and even commercial areas. There are rules about how big they can be and where they can go; read on to find out.

In some zones (we call them NR3, NR2, and NR1), you can build two ADUs. You can either have two attached ADUs or one attached and one detached one. But there's a catch - if you want to build a second one, it either has to be eco-friendly (we love green buildings!), or it needs to be an affordable home for economically struggling families.

In other places (like Neighborhood Residential Small Lots and low-rise multifamily zones), you can only have one ADU for each single-family home, rowhouse, or townhouse. But forget about ADUs in apartments in low-rise zones; they're not allowed. But they are good to go in apartments in RSL zones.

Here's some more good news: You don't need to provide parking for your ADU. But if you've got a parking spot already, you can't just get rid of it unless you can put it somewhere else on your property that follows the rules.

Another rule to note is you don't have to live on the property where you build the ADU, which is the case in some states.

Lastly, because your ADU will increase your building's sewer capacity usage, you'll have to pay a sewer treatment capacity charge.

The Seattle Department of Construction and Inspections will tell King County about your new ADU for the sewer charge, and you'll need to acknowledge these charges when you apply for your permit. But don't worry; they'll help you understand all the nitty-gritty details!

Rules and Regulations of ADUs in Seattle

Seattle Rules for Attached ADUs and Detached ADUs

Alright, let's break down the rules for building AADUs and DADUs in Seattle, and let's keep it super easy:

Attached ADUs in Seattle

Size Matters

If you're in a residential neighborhood, including those RSL spots, you can make your AADU a cozy 1,000 square feet. If you're in a low-rise area (that's where the buildings aren't super tall), you have to keep it a bit smaller - under 650 square feet.

Building Standards

Now, your AADU can't just be any old shack. It's got to follow all the Seattle housing rules. So everything, from the building's structure to the electrics, the heating, how the land's used, and even how you treat the special spots (environmentally critical areas and shorelines), needs to be up to mark.

Detached ADUs in Seattle

Room to Move

To install a DADU in a residential neighborhood, your yard needs to be a decent size - at least 3,200 square feet.

Size Again

Just like its attached cousin, a DADU can go up to 1,000 square feet in residential neighborhoods, including RSL areas, and up to 650 square feet in low-rise zones.

Building Standards

And, just like the AADU, your DADU must also play by the rules. It needs to follow all the Seattle rules that apply to single-family homes; this includes everything about how the building is made, the mechanics (think heating and AC), the wiring, and how energy-efficient it is.

If you are looking for manufactured homes, Seattle allows you to go with a lot of options. Below is a list of builders for your new prefab shed.

Seattle Rules for Attached ADUs and Detached ADUs

Best Prefab ADUs in Seattle


My KABIN has some good prefab homes in Seattle if you want a more traditional-looking prefab ADUs IN Seattle. MY KABIN deserves accolades for its outstanding offerings. The attention to detail and commitment to quality shine through in every aspect. The design and functionality of their products are truly impressive, providing customers with practical yet stylish solutions.

Emerald ADU Builders

Emerald is another good ADU Seattle builder that you can check out for customized or prefab homes in Seattle. Their professionalism and experience are evident, consistently delivering projects of outstanding quality.

The commitment they exhibit from the design stage through construction and completion is noteworthy. The team's proficiency is matched by their genuine dedication to crafting well-designed and harmonious living spaces.

Autonomous ADUs

Autonomous has the best tiny prefab homes. With a modern look and energy efficiency, their prefab homes under $100K will get you the most bang for your buck. Their commitment to quality and innovation is evident in every project they undertake. The professionalism and attention to detail they bring to the table are truly commendable.

Autonomous ADUs


So, there you have it, folks! Whether you want to add a bit of extra space to your pad, downsize, or just crave a bit of change, with ADU Seattle rules as simple as they are, and the help of tiny home builders, you will face no difficulty.

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