Architecture, Custom Residential
Designing & Permitting Backyard Buildings
What does it take to get your backyard building built? Whether its an accessory structure, a detached accessory dwelling unit, or a garage, a little gumption goes a long way. With careful planning and a little patience, you can have the backyard building of your dreams.
May 8, 2026
Backyard buildings are an increasingly sought-after approach to adding functionality and density to existing single-family homes. In our last post, we talked the differences between accessory structures, detached accessory dwelling units (DADUs), and garages, the three main types of backyard buildings.
In this post, we’re getting down to brass tacks, and looking at the ins and outs of designing, permitting, and building backyard buildings in Seattle.
Seattle’s take on accessory structures.
If you recall from the last post, accessory structures are typically the most straight-forward backyard building. Generally, they don’t even require a permit if the projected roof area does not exceed 120 square feet. That’s right: 120 square feet of freedom, no questions asked.
But let’s be honest. 120 square feet isn’t going to cut it for most of us. Once you go beyond that magic number, you’re looking at permit territory, but it’s usually a simple process that won’t make you question your life choices.
Size and Height Limitations
While limitations on space and height for accessory structures can be different depending on where you are in Washington, here’s what you’ll generally find when exploring size options.
- Maximum heights ranging from 12-16 feet.
- Floor area limits often around 750-1,000 square feet for detached structure.
- Setback requirements that keep structures a reasonable distance from property lines.
- Lot coverage limits that prevent you from paving paradise.
Permitting an Accessory Structure
Remember, you’re not dealing with complex utility connections, habitability requirements, or density calculations with accessory structures. In Seattle, accessory structures follow relatively predictable rules.
- In NR1, NR2, or NR3 zones you could put an accessory structure (a shed, for example) in the rear 25 feet of your property or the rear 20% of the lot depth, whichever is less.
- In NR1, NR2, and MR3 zones, you can put an accessory structure within 10 feet of the rear property line.
- In RSL zones, you can put an accessory structure within 10 feet of the property line abutting an alley.
Don’t worry if this sounds complicated. Your design team can help you out with the details.
The DADU revolution in Seattle.
Single-family-only zoning has officially ended in the City of Seattle. On May 27, 2025, the Seattle City Council passed CB 120969 to comply with RCW 36.70A.635, fundamentally changing how we think about residential density.
This interim zoning legislation has opened new possibilities for homeowners looking to add rental income or create multi-generational housing options.
Under the new rules, Seattle’s recent legislation facilitates Accessory Dwelling Unit (ADU) construction across neighborhoods. (Remember that a detached accessory dwelling unit is a type of ADU.)
The land use code now allows two ADUs on a single property, which can notably increase residential density. Minimum lot sizes for ADUs have been relaxed, removing a barrier that previously limited their development.
This is a game-changer. Where previously you might have been limited to one ADU (either an attached or a detached accessory dwelling unit, but not both), you can now potentially have both an attached and a detached accessory dwelling unit on the same property.
Size and Design Requirements
While Seattle has relaxed many restrictions, DADUs still come with specific requirements.
- Seattle does not require a specific minimum lot size for DADUs, but the lot must meet access, utility, and other practical requirements.
- Maximum size is typically 1,000 square feet or 40% of the floor area of the principal dwelling, whichever is less.
- Height restrictions vary by zone but generally allow for two stories.
- If building an ADU of any kind on your lot would require you to cut down any trees, it may not occupy more than 40% of your lot area.
Permitting a DADU
Let’s be clear: DADU permits are more complex than those for simple accessory structures.
- Project have full structural engineering requirements.
- You’ll need to address utility connections, like sewer, water, electrical.
- They need to meet habitability codes.
- They need to meet accessibility requirements.
- They potentially require SEPA (State Environmental Policy Act) review.
The process typically takes several months and requires more upfront investment in design and permitting. However, many homeowners find the long-term benefits worth the initial complexity.
The basics of garage regulations.
In Seattle, most new and remodeled garages require only a subject-to-field-inspection (STFI) permit. To qualify, your garage needs to be 750 square feet or less, a single story without a roof deck, and with beams spanning 14 feet or less. This makes garages some of the easier structures to permit and build.
But here’s where it gets interesting: the moment you want to do anything beyond parking cars and storing stuff, the rules become more complex.
When planning garage projects there are a few important things to consider.
Fire Safety
Attached garages and those within three feet of the house need additional fire protection, including gypsum board on interior walls and fire-rated assemblies at openings.
Structural Requirements
Converting a garage often requires structural modifications to meet habitability codes, including proper insulation, ventilation, and sometimes foundation work.
Utility Access
Adding plumbing to a garage can be straightforward or extremely complex, depending on the location of existing utility lines.
Parking Requirements
Some jurisdictions have minimum parking requirements that you’ll need to address if you’re converting your only garage to living space.
Navigating the permit process for backyard buildings.
Understanding the theory behind these building types is one thing; getting permits and building them is another. Here’s what you need to know about navigating the bureaucratic waters. (Pst! Your architect is a great resource to help you prepare for your own project!)
Know Your Jurisdiction
The first and most important step is understanding which municipality has jurisdiction over your property. Seattle proper operates under one set of rules, but step into Shoreline, Lake Forest Park, or any other nearby city, and you’re dealing with entirely different regulations.
Each jurisdiction has its own setback requirements, height limitations, size restrictions, design review process, permit fees, and timelines.
The Pre-Application Meeting
A pre-application meeting or consultation with your jurisdiction is an often free or low-cost opportunity to sit down with city planning staff to discuss your project before investing time and money into design and permitting.
These meetings can help you confirm zoning requirements, understand potential for complications, get guidance on the permitting processes, and identity any unique site restraints for your project.
A good pre-application meeting can save you months of back-and-forth during the actual permit process.
Design Review
Depending on your location and project scope, you might encounter design review requirements. Seattle’s design review process, for example, can apply to larger DADU projects or projects in certain neighborhoods.
Design review isn’t just about aesthetics. It’s about ensuring your project fits within the broader neighborhood context. Design review considers building scale and massing, materials and colors, landscape and site design, and parking and access.
While design review can add time and complexity to your project, it also provides an opportunity to get community input and avoid conflicts with neighbors.
Remember: the goal isn’t to outsmart the system or find clever workarounds. It’s to work within the existing framework to create spaces that enhance your property while respecting the broader community context.
The Economics of backyard buildings.
Let’s talk money. All backyard buildings come with a cost, with the cost of construction varying significantly depending on the project type.
Accessory Structures
The most cost-effective option, simple sheds run $20-50 per square foot for basic construction. More elaborate workshops or studios might run $75-150 per square foot.
DADUs
Significantly more expensive due to the need for full utilities, habitability features, and more complex permitting. Expect costs in the range of $200-400 per square foot, depending on finishes and site complexity.
Garages
Somewhere in between, with basic detached garages running $100-200 per square foot. Garage conversions can vary widely depending on the scope of work required.
Permit and Soft Costs
Don’t forget about the costs beyond construction.
- Permit fees (ranging from hundreds to thousands of dollars).
- Design and engineering fees.
- Utility connection costs.
- Potential impact fees or other municipal charges.
For DADUs especially, soft costs can represent 15-25% of the total project budget.
Return on Investment
While accessory structures and garages primarily add value through improved functionality and storage, DADUs offer the potential for ongoing rental income. In Seattle’s rental market, a well-designed DADU can (at the time of this post) generate $1,500-3,000+ per month in rental income.
Navigating the world of accessory structures, DADUs, and garages in Western Washington requires patience, planning, and a willingness to engage with local bureaucracy. But for homeowners willing to do their homework, these projects can add significant value and functionality to their properties. Research your local regulations thoroughly, and don’t hesitate to engage with a design team and municipal staff early in the process.
Now go forth and build responsibly! Your future self (and your neighbors) will thank you.
