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Two accessory dwelling units (ADUs) can have the same square footage and very different construction costs. One may use an existing basement, while the other needs excavation, a foundation, drainage, utility trenches, and a complete detached structure. So, how much does it cost to build an ADU in Seattle, and what should your budget include beyond the building itself?
I understand why Seattle homeowners look for one quick number. Building an ADU is a major investment, and you want to know whether the idea is realistic before paying for plans. The problem is that an online average cost cannot see your slope, sewer route, electrical panel, access, or existing structure.
I’m Leo, founder of Alta Group Remodeling. When we plan an ADU, I begin with the property and the reason you want the space. In this guide, I will walk you through what we need to build, what could change, and which decisions to make before design, permitting, and construction pick up speed.
Start With the Type of Seattle ADU
An attached accessory dwelling unit, or AADU, is incorporated into or connected to the main house. It may be a basement conversion, attic conversion, addition, or reconfigured living space. A detached accessory dwelling unit, or DADU, is a separate structure often called a backyard cottage.
A conversion may already have a roof, walls, and foundation, but that does not automatically make it simple. We still examine moisture, ceiling height, egress, insulation, fire separation, plumbing, electrical capacity, and structural conditions.
A new DADU is closer to building a compact custom home. It needs its own foundation, envelope, roof, windows, heating, ventilation, plumbing, electrical work, and finished living areas. That is why I first ask how you will use the unit and what your Seattle property gives us to work with.
Our Current Seattle ADU Cost Planning Ranges
These are our current Seattle ADU planning ranges, not fixed packages or Seattle-wide averages. Each row represents a different kind of ADU, so do not add them together. Our detached ADU guidance also uses approximately $400 to $650 per square foot as broad planning context before unusual site work.
| ADU scope | Starting point | Current planning range |
|---|---|---|
| Garage or basement ADU conversion | $125,000 | $150,000 to $300,000+ |
| Attached ADU | $175,000 | $225,000 to $450,000+ |
| Detached ADU or backyard cottage | $300,000 | $325,000 to $550,000+ |
How Much Should You Budget Beyond the ADU Construction Quote?
First, find out what the quoted number actually covers. An ADU construction quote may not include every cost needed to move from an idea to a legal, finished dwelling. The construction cost and the total project cost are not always the same number, so an early quote should not automatically be treated as the all-in cost. Depending on the proposal, you may need to budget separately for design, engineering, surveys, permit and utility fees, taxes, testing, site preparation, service upgrades, landscaping restoration, financing, and temporary storage or access arrangements. Selected materials above an allowance and homeowner-approved changes can also move the final cost.
I do not recommend adding an arbitrary percentage and assuming you are protected. The better approach is to identify known costs, allowances, exclusions, and unresolved conditions separately. A reserve should reflect what remains concealed or unconfirmed on your property. That gives you a useful cost breakdown instead of a comforting number that may fall apart later.
What Does a Detailed Seattle ADU Cost Estimate Include?
A detailed ADU estimate should show more than one total price. I want homeowners to see how the budget is divided between design, site preparation, construction, utilities, and project coordination. Each proposal should also explain what is included, what remains an allowance, and which costs may change as the project develops. Here is what I recommend reviewing before approving an estimate.
| Estimate area | What should be defined |
|---|---|
| Design and approvals | Drawings, engineering, survey, permit coordination, and agency fees |
| Site work | Access, clearing, excavation, drainage, foundation, and restoration |
| Building shell | Framing, roofing, windows, doors, insulation, and exterior finishes |
| Utilities and systems | Sewer, water, electrical, heating, ventilation, and required upgrades |
| Interior construction | Kitchen, bathroom, drywall, flooring, cabinetry, tile, paint, and fixtures |
| Project operations | Supervision, protection, deliveries, waste removal, inspections, and cleanup |
| Commercial terms | Allowances, taxes, exclusions, changes, payment schedule, warranty, and closeout |
Starting prices represent entry points for certain project scopes, while our planning ranges reflect the broader budgets homeowners should generally expect. Neither figure is a guaranteed quote.
If one proposal includes utility trenching, ADU permit coordination, and finish installation while another does not, the totals are not comparable. I want you to understand where the number came from before you sign, not after the yard is open.
Which ADU Design Decisions Have the Biggest Impact on Cost?
The first major decision is the ADU type. A garage conversion or basement conversion is different from constructing a detached backyard cottage. After that, layout simplicity matters. Moving the kitchen and bathroom closer to workable utility routes may reduce piping and coordination. Complicated rooflines, large openings, custom structural work, extensive glazing, and highly customized cabinetry can add design, fabrication, and labor costs.
Finish level matters, but it is not always the largest cost driver. I would rather simplify a countertop or tile selection than compromise drainage, waterproofing, ventilation, or an in-wall plumbing component. If the ADU is for an aging parent, accessibility and daily comfort may deserve more of the budget than decorative upgrades.
When Can Existing Utilities Support an ADU Without Major Upgrades?
Existing services may work when they have adequate capacity, are in suitable condition, and can reach the unit through a practical route. We review the electrical service and panel capacity, water service, sewer location and condition, drainage, heating approach, and the distance between the main house and proposed ADU. When those pieces line up, utility connections may avoid major service upgrades.
An available breaker space does not by itself confirm electrical capacity. A nearby sewer line does not guarantee the required slope or a workable connection. Licensed professionals, utilities, and the permitting authority may need to confirm the final approach.
Utilities become more expensive when service equipment must be upgraded, trenches cross hardscape, lines run around structures or trees, or the route requires pumping or specialized work. This is why utility research belongs in feasibility planning, not after the floor plan is finished.
How Much Can Site Preparation Add to a Backyard Cottage Budget?
At Alta, we do not publish one flat site-preparation allowance because the work can range from straightforward access and excavation to extensive drainage, retaining, soil, or foundation requirements. Unusual site work can push a DADU beyond the top of our published planning range rather than adding one predictable amount.
The largest site cost drivers are restricted equipment access, slope, unsuitable soil, tree constraints, demolition, long utility trenches, drainage work, retaining conditions, and restoration of fences, paving, or landscaping. A small backyard cottage can still carry significant cost because crews and materials must reach it and the completed building must be supported and drained correctly. When you build a DADU, the work outside its walls is part of the construction project too.
Before promising a site number, we review the property and identify which conditions require a survey, engineering, utility research, or further investigation.
Seattle ADU Permits and Current 2026 Fees
The Seattle Department of Construction and Inspections states that an ADU must be established through the permit process. The City’s ADU guidance also explains Seattle ADU rules and its pre-approved DADU plans. Those plans can make review faster and more predictable, but the site plan, utilities, property conditions, and DADU construction are still project-specific.
For 2026, SDCI increased most fees by 6.5 percent and set its base hourly rate at $292. Construction and Master Use Permit fees increased by 18 percent. The SDCI fee summary explains that the hourly rate is part of the calculation, not the total ADU permit price.
King County’s current capacity charge is $77.99 per residential customer equivalent per month for applicable connections made in 2026. Its capacity-charge guidance lists an ADU at 0.59 RCE, or $46.01 per month, when the charge applies. The property and sewer connection determine applicability.
At Alta, we coordinate the permitting and engineering included in the approved scope. Final decisions and approvals remain with the responsible agencies.
How We Build an ADU Budget at Alta
I separate the budget into required construction, worthwhile performance upgrades, and wish-list details. We define the ADU type, layout, site, utilities, and permit path before refining finishes.
Depending on the project, we may offer a separate planning package covering design, engineering, a land survey, utility research, and feasibility work. This phase can cost approximately $10,000, although the exact price and included services depend on the approved scope.
What Our Backyard ADU Shows About Cost
Our Backyard ADU is a useful example of how a compact detached structure can fit into an established residential property. Its exterior features wood siding, a corrugated roof, and multiple windows.
A project like this involves more than the visible building. Site access, foundation requirements, drainage, utility connections, structural work, and interior construction can all affect the budget. The specific scope depends on the property and approved plans.
That is why I do not estimate DADU costs based on square footage alone. Even a relatively small backyard cottage requires careful planning around the site, building systems, and intended use.
Plan Your Seattle ADU With Alta Group Remodeling
At Alta, we design and build attached ADUs, DADUs, garage and basement conversions, and guest suites across Seattle, Everett, the Eastside, King County, and Snohomish County.
Our first consultation is a conversation about your property, goals, concerns, and working budget. We do not charge for that initial consultation. If you continue, we define and price additional feasibility, design, engineering, permitting, or investigation before that work begins.
Schedule a complimentary in-home consultation or call (206) 773-7361 to discuss the type of ADU your property may support and what it could take to build it.
FAQ
How much does it cost to build an ADU in Seattle?
Our current attached ADU range is $225,000 to $450,000+, starting at $175,000. The current DADU cost range is $325,000 to $550,000+, starting at $300,000. Site conditions, utility work, design, permits, and finishes determine where an ADU project falls within or beyond those ranges.
Is a basement or garage conversion always less expensive?
No. Moisture, slab, ceiling height, egress, utilities, insulation, and fire-separation work can significantly affect a conversion.
Can a pre-approved DADU plan reduce the cost?
It may reduce some design work and create a more predictable review path. Site planning, utilities, permits, materials, and construction remain property-specific.
Can an ADU generate rental income or increase property value?
A permitted ADU may support rental use and marketability, but demand, expenses, financing, regulations, and resale results vary. We do not guarantee income or return on investment.
How do I get an accurate ADU estimate?
Begin with a property review. We evaluate the intended use, ADU type, existing conditions, access, utilities, permit path, layout, and finish expectations before defining the scope and cost.
What costs are typically excluded from an initial ADU construction quote?
An early construction quote may not include design, engineering, surveys, permit fees, utility upgrades, taxes, site preparation, or landscaping restoration. The exclusions depend on the contractor and project scope. I recommend reviewing these costs separately before comparing estimates so you understand the total investment, not just the building price.
