More Kirkland homeowners are exploring the idea of adding space to their property, whether through an extension of their current home or a separate unit on the same lot. Attached additional dwelling units (ADU) or a detached ADU (DADU) can both create valuable living space, generate rental income, accommodate family members, and increase your property’s long-term value. The right choice depends on your lot, your budget, and your goals.
In this guide, we’ll explain the key differences between attached and detached ADUs, compare their costs and design considerations, outline the regulations that may affect your project, and help you determine which option makes the most sense for your home.
What Is an ADU and What Is a DADU?
Both an ADU and a DADU add independent living space to your property without requiring you to buy a larger home or move. The key difference is simple: an ADU is attached to your existing home, while a DADU is a completely separate building on the same property.
An ADU (Accessory Dwelling Unit) is built within or attached to the primary residence. Common examples include basement apartments, above-garage units, home additions, or conversions of existing interior space. In every case, the ADU shares walls or structure with the main home, making it part of the existing building.
A DADU (Detached Accessory Dwelling Unit) is a standalone structure located elsewhere on the property. Backyard cottages, guest houses, and carriage houses are all examples of detached ADUs. Because a DADU is its own building, it requires a separate foundation, exterior structure, roof, and utility connections. That typically makes it more expensive to build than an attached ADU, but it also offers greater privacy, flexibility, and independence for both the occupants and the main household.
| ADU | DADU | |
|---|---|---|
| Attached to primary home? | Yes | No |
| Typical forms | Basement suite, above-garage unit, Junior ADU | Backyard cottage, carriage house, guest house |
| Separate foundation required? | No (uses existing structure) | Yes |
What Rules Govern ADUs and DADUs in Kirkland?
Kirkland’s ADU rules fall into six categories. Confirm current regulatory requirements with the City of Kirkland Planning Department before investing in design.
Zoning. Your lot's zoning designation and size determine whether you qualify for an ADU, a DADU, or both. Under Washington's 2023 ADU law (House Bill 1337), Kirkland now allows up to two accessory units on most single-family lots, configured as one attached and one detached unit or two of the same type. Eligibility still depends on your specific lot, so not every property qualifies for every configuration.
Permitting. Both types require building permits. A detached structure typically involves a more complex permit process than an attached conversion because it is treated as new construction. Kirkland also offers a pre-approved DADU program that can shorten the design and permitting timeline for qualifying lots.
Setbacks. How close a structure can sit to property lines differs by type. DADUs, as freestanding buildings, generally face more involved setback requirements than attached ADUs, and proximity to other structures can trigger additional building requirements.
Size limits. Both types have maximum allowable square footage relative to the primary home or lot size. The City of Kirkland sets these limits, and they are subject to change.
Utility connections. DADUs typically require separate water, sewer, and electrical connections. These utility tie-ins are one of the most common budget surprises for homeowners.
Parking. Historically, adding a dwelling unit could trigger an additional off-street parking requirement. State law now waives that requirement for ADUs near frequent transit, but whether it applies still depends on your lot, so confirm your specific situation.
Rules change. In 2024, cities across Washington, including Kirkland, updated their zoning to comply with House Bill 1337, making ADU and DADU construction more accessible across the Eastside. The reforms generally allow more units per lot and remove older restrictions such as statewide owner-occupancy mandates, but the specifics continue to evolve and vary by property. Always confirm current requirements directly with the City of Kirkland Planning and Building Department before starting your project.
What Drives the Cost of an ADU vs. a DADU?
Every project is different, and an accurate budget requires a site-specific assessment. That said, six factors consistently drive cost in one direction or the other.
Attached vs. detached structure. A DADU requires a full foundation, a complete exterior shell, and separate utility connections. An ADU that converts existing space, such as a basement or garage, starts with a structure that already exists, which typically reduces the scope of new construction.
Size. Larger units cost more to build. Both types have code-defined size limits, and building to the maximum allowable footprint increases cost proportionally.
Site conditions. Slope, soil type, drainage, and the location of existing utilities all affect what it costs to build. A flat lot with utilities nearby is a simpler project than a sloped lot requiring grading and extended utility runs.
Finish level. Flooring, cabinetry, appliances, and fixtures have a meaningful impact on total cost. Finish selection is a key decision point: the right level depends on whether you are optimizing for long-term livability or maximum resale return.
Utility connections. DADUs require new or upgraded connections for water, sewer, and electrical service. These connections add both cost and permitting complexity, and they catch many homeowners off guard.
Permit and design fees. Both types require architectural drawings and building permits. Fees vary by jurisdiction and project scope, and they belong in your budget from day one.
Ready to understand what an ADU or DADU would cost on your specific lot? Contact us for a straightforward conversation about your property and goals.
Matching the Right Type to Your Goals
Your lot will constrain your options, but your goals should guide the decision. Here is how the two types tend to perform across the four most common homeowner motivations.
| Goal | ADU Fit | DADU Fit |
|---|---|---|
| Rental income | Good; generates income with lower upfront construction scope | Strong; full privacy and a separate entrance often command stronger tenant interest |
| Multigenerational living | Strong for families who want easy access between units | Strong for families who prefer more independence and separation |
| Home office or studio | Works well; lower cost to build if converting existing space | Offers the clearest separation between work and home life |
| Resale value | Adds appraised value as an income-generating asset | Often recognized by appraisers as a stronger standalone income asset |
Both ADUs and DADUs are recognized by appraisers as income-generating assets that raise resale value on the competitive Eastside market. The stronger performer for your specific property depends on what the lot actually allows.
ADU vs. DADU: Side-by-Side Comparison
| Dimension | ADU | DADU |
|---|---|---|
| Definition | Secondary dwelling attached to or within the primary home | Fully freestanding secondary dwelling on the same lot |
| Physical relationship | Connected to or integrated with the primary structure | Physically separate; own foundation and exterior |
| Typical use cases | In-law suite, rental unit, home office conversion | Backyard cottage, guest house, carriage house |
| Relative cost | Generally lower when converting existing space | Generally higher; full new construction scope required |
| Utility connections | Typically shared or extended from the primary home | Typically requires new or upgraded separate connections |
| Permitting complexity | Moderate; depends on conversion scope | Higher; treated as new construction |
| Privacy for occupants | Moderate; shares walls or structure with primary home | High; fully separate structure |
| Best fit for | Cost-conscious projects, multigenerational access | Maximum rental appeal, full independence, resale strength |
Specific rules and costs vary by lot and jurisdiction. Confirm all current requirements with the City of Kirkland Planning Department before planning your project.
Five Common Planning Mistakes to Avoid
Assuming your lot qualifies without checking zoning. Zoning designation and lot size determine eligibility for each type. Confirm before investing in design or architectural drawings.
Underestimating utility connection costs. Especially for DADUs, new or upgraded water, sewer, and electrical connections are a significant budget item. These are among the most common surprises for homeowners.
Designing to a footprint that the lot cannot support. Lot coverage limits and setback requirements can make a planned footprint impossible. Confirm your constraints before finalizing the design.
Choosing a type based on cost alone. The lower-cost option is not automatically the right option if it does not serve the intended use. A basement ADU is less expensive than a backyard cottage, but it may not meet your rental or privacy goals.
Starting design before understanding permit requirements. Permit requirements should shape the design from the beginning. Working with a builder who knows Kirkland's permitting landscape saves time and avoids costly redesigns.
An experienced local builder can help you work through these questions before they become expensive problems. See how our process works from the first conversation to the final punch list.
Frequently Asked Questions About ADUs and DADUs in Kirkland
Can I build a DADU in Kirkland?
Many Kirkland lots are eligible for a DADU, but eligibility depends on your lot’s zoning designation, size, and existing structures. Recent code updates have made more properties eligible than in previous years. Confirm your lot’s current eligibility directly with the City of Kirkland Planning Department before proceeding.
What are the rules for ADUs in Kirkland?
The main regulations determine whether your property qualifies for an ADU, how many ADUs are allowed, the maximum unit size (generally up to 1,200 square feet), required setbacks and height limits, lot coverage and floor area requirements, utility and service connections, and whether off-street parking is required based on your property’s proximity to transit. Because Kirkland has updated its ADU code in recent years to comply with new Washington housing legislation, homeowners should always verify the latest requirements with the City’s Planning and Building Department before beginning a project
Which is better for rental income, an ADU or a DADU?
Both types generate rental income and are recognized by appraisers as income-generating assets. A DADU, as a fully separate structure with its own entrance and no shared walls, often appeals strongly to tenants who prioritize privacy. An attached ADU can also be a productive rental unit, particularly when lot constraints make a detached structure difficult to permit. The right answer depends on your specific lot and what the zoning allows.
How much does it cost to build an ADU or DADU in Kirkland?
Costs vary significantly based on whether you are converting existing space or building new, plus site conditions, size, finish level, and utility connection requirements. As a general guide, ADU conversions on the Eastside tend to start around $200,000, while a new detached DADU more commonly falls in the $400,000 to $650,000 range because it requires a full foundation, exterior shell, and separate utility connections. These are broad market figures, not a quote. The only reliable way to understand what a project would cost on your lot is a site-specific assessment with a builder who knows the local permitting and construction landscape.
What is the difference between an ADU and a DADU in Kirkland?
An ADU (Accessory Dwelling Unit) is a secondary dwelling that is attached to or part of the primary home, such as a basement suite or above-garage unit. A DADU (Detached Accessory Dwelling Unit) is a fully freestanding structure on the same lot, such as a backyard cottage. Both are legal secondary dwellings in Kirkland, but they are governed by different rules around setbacks, utility connections, and permitting complexity.
Ready to Build an ADU or DADU in Kirkland?
Choosing between an ADU and a DADU comes down to three things: what your lot allows, what you want the space to accomplish, and what budget you are working with. The best next step is a conversation with a builder who knows Kirkland’s permitting landscape and has handled the full range of complexity these projects involve.
Tennyson Homes is a family-owned general contractor building ADUs and DADUs across Kirkland, Bellevue, and Mercer Island, with over 34 years of experience on the Eastside since 1992. We manage the full project scope from permit coordination to final punchlist, and we work with your chosen architect or connect you with our vetted network of the region’s top talent to create a unified project team. You can also browse completed projects to see the quality and range of work we deliver.
