Can You Build an ADU or DADU in Your Backyard?

What Are ADUs and DADUs?
Hey, Charlie. Can I actually build a tiny house in my backyard? Are you wondering if you can build an ADU or DADU on your property? Maybe you’ve heard about these, but you aren’t sure if your lot qualifies or what the process involves. Well, you’re in the right place. Today, we’re diving into everything you need to know about building an ADU or DADU—from understanding what they are, if your property is eligible, to navigating permits and estimating cost. Whether you’re looking to generate rental income, create space for family members, or just add value to your property, this guide will help you determine if an ADU or DADU is possible. Let’s get started.

Hi, my name is Charlie and I am with Armada Design and Build in Bellevue, Washington. We specialize in turning your dreams into reality. From custom homes to full renovation, visit our office to explore your options and schedule a consultation. Armada Design and Build—building your tomorrow today.

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First things first, an ADU stands for accessory dwelling unit. It’s essentially a secondary housing unit on your property. Now, there are a few different types. An ADU is connected to the main home, which can be achieved by converting an existing part of your home or adding an extension onto the house—or a combination of the two. Take some space you have, build a small addition, and turn that into an ADU.

Second is a DADU, which is a detached accessory dwelling unit. This is a separate structure from the main house out in the backyard, like a cottage or a detached garage. The beauty of these units is their versatility. They’re fully functional, self-contained living spaces with their own kitchen, bathroom, and sleeping area, but they are typically smaller than the primary residence. To be fully functional, though, you need cooking space, a bathroom, living area, and controlled heating and cooling. While not required, to me, laundry makes sense—if you’re going to be able to do everything, laundry is one of the things you need to be able to do.

Property Eligibility & Zoning Rules
Can your property accommodate an ADU? This is often the biggest question, and the answer depends on a number of factors specific to your situation, your property, and where you live. The first thing to understand is zoning requirements, or land use as it’s called in some places. First and foremost, your property must be zoned in a way that ADUs are allowed. Not all residential zones permit them. This is the very first thing to check to get a yes or no. Do some zoning research with your local building department. Is it the city or the county? Where do you get building permits from? They dictate what can be built in certain areas—business zones, transportation, residential. What specific zone is the property in? R3, R5, whatever it is.

Once you understand your zoning, the next most important thing—at least for us here in the Northwest—is your lot size. Understand how big your lot is in your zoning area. How many square feet will dictate a lot about whether you can build an ADU or DADU, or what size it can be. Most ADU/DADU regulations are based on the zoning and the lot size. In our experience, these are two of the main factors that determine what you are allowed to build.

Lot Size, Coverage & Setbacks
Lot coverage is another one. In our area, there are limits on how much of your lot can be covered by structures, driveways, patios, and so on. This is another aspect you’ll need to understand as part of determining what you can build.

Setbacks are also important. These are the required distances from the property line. You can’t typically build right on the property line. You must have a buffer between the property line and the structure. You’ll need enough space to accommodate the ADU while meeting those required setbacks from property lines, streets, and other structures.

Access, Parking & Utilities
Access requirements are another consideration. Many jurisdictions require ADUs—and certainly DADUs—to have their own entrance, and sometimes even dedicated parking.

Utility requirements can often be significant but are an overlooked part of building a DADU especially. DADUs being completely separate buildings are a different story than ADUs. Getting electricity, water, sewer, gas if available in your area, and of course internet, out to the DADU can often be costly and somewhat difficult to manage. If you have a septic system, the complication and cost factor can increase even more.

Popular Uses for ADUs/DADUs
Some popular uses for ADUs and DADUs—why are homeowners interested in adding these? Based on what we’re seeing with our clients, here are the three main reasons.

Rental income is a big one. Adding an ADU or DADU can generate significant monthly income, especially in areas with high housing demand. We’ve had clients cover a substantial portion of their mortgage with ADU rental income.

Family housing is another. Many homeowners are building ADUs to keep family close while maintaining some privacy—whether it’s aging parents who need some support but still want independence, or young adult children saving for their own home.

Extra space is the third. Some clients simply want additional living space: a home office separate from the main house, a studio, a guest suite, or even just a personal retreat.

Permits & Local Regulations
Permits and regulations are one of the most complicated parts of the process. If your property meets the basic requirements, the next step is understanding the permitting process. Regulations vary significantly between municipalities. Some are very much for it, making permitting easier. Some are less enthusiastic, making permitting more difficult. Be prepared for a process that can typically take 3 to 6 months, but can stretch to 9 or 12 months in more complex cases or stricter jurisdictions.

Some jurisdictions have specific design requirements for ADUs and DADUs, requiring them to match or complement the main dwelling. Some areas require the owner to live in either the main house or the ADU/DADU, which can affect long-term plans. Others don’t allow for short-term rentals, so be aware if that’s part of your plan. Short-term rental rules are also changing quickly in some places.

ADU/DADU Cost Ranges
Cost is another key factor. Prices can vary widely based on what you build, how big it is, and the finishes. Many people think an ADU or DADU is a $50,000 to $150,000 project. They are often surprised to learn costs are significantly higher. Converted spaces like basements or garages are usually the lowest cost, often between $100,000 and $200,000 depending on the level of rework.

Attached ADUs where you are building new space are more expensive, since you’re not only building the inside but also the outside structure. These typically start at $225,000 to $300,000 and can reach $400,000 depending on scope and size.

Detached DADUs are usually the most costly since they’re completely new structures. They often start in the high $200,000s in our area and can reach $600,000 to $700,000 for larger or more complex builds. Most land in the $350,000 to $475,000 range.

Converting Existing Structures
Some people want to convert a detached garage or shed. While doable, the rework to bring these structures up to code can cost nearly as much as building new. Costs also fluctuate depending on site work like tree removal, excavation, foundations, and utility connections. Septic systems in particular can add significant cost.

Hidden Costs to Watch Out For
Finishes also impact cost. Windows, roofing, flooring, cabinets, tile—everything you choose matters. Site accessibility is another factor. If equipment can’t easily get into the backyard, costs go up. Local labor and material prices also play a role.

A word of caution: many companies advertise pre-packaged ADU designs with published costs, but often those prices don’t include excavation, foundations, or utility costs. These hidden costs can add 25–35% to the published price. So be sure you know exactly what is and isn’t included.

Final Advice & How Armada Can Help
Can you build an ADU or DADU on your property? The answer depends on your specific situation, local regulations, and property characteristics. While the process can be complex, the benefits—additional income, flexible living space, housing for family members—often make it worth exploring. Just be aware and understand what it will take.

Here at Armada Design and Build, we’ve helped countless homeowners navigate the ADU/DADU process from initial feasibility assessment to completed construction. We understand local codes and requirements, the potential pitfalls, and the best practices we’ve developed over time to ensure your project succeeds if your property qualifies.

Please leave your comments or questions below and we’ll do our best to respond. Thank you very much.