“How much does an ADU cost per square foot?” is the first question most Seattle homeowners ask. It's a reasonable starting point for comparing builders, but cost per square foot is a misleading metric without context. Two builders quoting the same $/sqft number can deliver vastly different projects depending on what's included, what's excluded, and what assumptions they're making about your site.
This guide provides current 2026 cost data for ADU construction in Seattle and King County, explains why cost per square foot varies so widely, and shows you how to evaluate bids beyond the headline number.
For a complete cost breakdown specific to your property, request a free feasibility study. We'll analyze your lot conditions, utility access, and building constraints to provide a realistic cost estimate — not a generic range.
Average Cost Per Square Foot by ADU Type
These ranges reflect all-in costs for ADU construction in Seattle and King County as of early 2026, including design, permitting, and construction. Actual costs depend on site conditions, finishes, and complexity.
Detached ADU
$500 – $700+/sqftNew construction from the ground up. All-in total starts at $400,000. Quotes under $400K almost always exclude permits, site prep, or utility connections.
Attached ADU
$400 – $500/sqftShares a wall with the primary home. Similar per-sqft cost but smaller typical size keeps total lower.
Garage Conversion
$400 – $500/sqftConverts an existing garage structure. Existing shell helps, but plumbing, electrical, HVAC, and finishes still add up.
Basement Conversion
$400 – $500/sqftConverts existing basement space. Scope of work is significant — egress, waterproofing, plumbing, and finishes drive costs.
Ranges based on Seattle ADU Builders project data, 2024-2026. Your project may fall outside these ranges based on site-specific conditions.
Why Cost Per Square Foot Is Misleading
Cost per square foot is a useful shorthand, but it hides an important reality: fixed costs don't scale with size.
Every ADU project has costs that remain roughly the same whether you build 400 sqft or 1,000 sqft:
- Design & engineering (Seattle licensed architect): $18,000 – $35,000
- Seattle DCI permits & fees: $20,000 – $50,000 (scales with project valuation)
- Utility connections (Seattle City Light + SPU): $15,000 – $40,000
- Kitchen & bathroom (mid-grade Seattle finishes): $35,000 – $65,000
- Mini-split HVAC system (Mitsubishi, Daikin): $6,000 – $12,000
The Math in Action — Seattle Market 2026
400 sqft DADU (studio)
~$90,000 fixed costs + $220,000 size-dependent costs = $310,000–$380,000 total
= $775–$950/sqft
800 sqft DADU (1-2 bedroom)
~$90,000 fixed costs + $380,000 size-dependent costs = $470,000–$600,000 total
= $590–$750/sqft
The 800 sqft DADU costs $185–$200/sqft less than the 400 sqft studio — not because it's cheaper to build per linear foot of wall, but because fixed costs (Seattle DCI permits, design, utility connections) are spread over twice the floor area.
What Drives ADU Costs Up
These factors can push your project toward the higher end of the cost range — or beyond it. Understanding them helps you budget realistically and make informed design decisions.
Hillside or sloped lots requiring engineered foundations and retaining walls
Extended utility runs (sewer, water, electrical) over 50 feet from connection points
Premium finishes: quartz countertops, hardwood floors, custom cabinetry
Complex designs: multi-story, irregular footprints, extensive glazing
Permitting challenges: critical areas, watershed restrictions, design review
Poor soil conditions requiring pilings, soil remediation, or enhanced drainage
Second-story ADUs requiring structural upgrades to existing buildings
What Drives ADU Costs Down
These factors work in your favor, bringing costs toward the lower end of the range. If several apply to your property, you're in a strong position for an affordable ADU project.
Flat, accessible lots with minimal grading required
Close proximity to utility connections (sewer, water, power)
Standard finishes: laminate counters, LVP flooring, stock cabinetry
Simple rectangular footprint with straightforward roof design
No critical area overlaps or watershed restrictions
Existing infrastructure to leverage (conversions vs. new construction)
Efficient floor plan that minimizes hallways and wasted space
Cost Breakdown by Category
Understanding where your money goes helps you evaluate bids and make smart decisions about where to invest and where to economize. Here's how a typical detached ADU budget breaks down:
| Category | % of Total | Includes |
|---|---|---|
| Design & Architecture | 5 – 8% | Architectural plans, structural engineering, energy calculations, design review. |
| Permitting & Fees | 3 – 5% | Building permit fees, utility connection fees, impact fees, plan review. |
| Site Work & Excavation | 8 – 12% | Grading, excavation, utility trenching, temporary access, erosion control. |
| Foundation | 10 – 15% | Footings, foundation walls, slab or crawlspace, waterproofing. |
| Framing & Exterior | 15 – 20% | Wall framing, roof structure, sheathing, siding, roofing, windows, doors. |
| Mechanical / Electrical / Plumbing | 15 – 20% | HVAC (mini-split), electrical service/panel, plumbing, water heater, fixtures. |
| Interior Finishes | 20 – 25% | Drywall, paint, flooring, cabinets, countertops, trim, fixtures, appliances. |
| Landscaping & Site Restoration | 3 – 5% | Grading restoration, plantings, pathways, deck/patio, fencing. |
Percentages are approximate and vary by project. Based on Seattle ADU Builders project data for detached ADUs.
How to Read an ADU Bid
Not all bids are created equal. A lower number on paper doesn't mean a cheaper project if major costs are excluded. Here's what to look for:
Should Be Included
- Architectural design and engineering
- All construction labor and materials
- HVAC, electrical, plumbing rough-in and trim
- All interior finishes and appliances
- Permit application management
- Project management and inspections
Often Excluded (Ask!)
- Seattle DCI permit fees ($20,000 – $50,000 for a DADU — scales with project valuation)
- Utility connection fees ($10,000 – $25,000)
- Landscaping and site restoration ($5,000 – $15,000)
- Geotechnical or environmental studies
- Deck, patio, or exterior living spaces
- Furniture, window coverings, exterior lighting
When comparing bids, add excluded items back in to get the true project cost. A bid of $280,000 that excludes $40,000 in utility fees, permitting, and landscaping is actually a $320,000 project. Learn more about comparing bids in our contractor hiring guide.
Our Pricing Approach: Transparent, Fixed-Price Bids
At Seattle ADU Builders, we provide fixed-price contracts with a detailed line-item breakdown. Our bids include design, permitting management, and construction — and clearly list what's excluded so there are no surprises. We believe you should know the full project cost before you commit.
Our pricing starts with a free feasibility study that analyzes your specific property. Because site conditions, utility access, and lot constraints vary widely across Seattle and King County, generic $/sqft numbers only tell part of the story. We provide a cost estimate tailored to your lot.
Visit our pricing page for current base rates by ADU type and size, or see our King County cost breakdown for detailed local data.
Seattle DCI Permit Fees: What You're Actually Paying
Seattle DCI permit fees are one of the most frequently underestimated line items in ADU budgets. Unlike many jurisdictions with flat-rate ADU permit fees, Seattle calculates building permit fees based on project valuation — i.e., the total construction cost. For a $500,000 DADU, permit fees alone can reach $25,000–$40,000 before accounting for utility connection fees. Here is what the Seattle permit fee structure typically looks like for ADU projects:
| Fee Type | Administering Agency | Typical Range (DADU) |
|---|---|---|
| Building permit fee | Seattle DCI | $8,000–$20,000 (scales with valuation) |
| Plan review fee | Seattle DCI | $3,000–$8,000 |
| Electrical permit | Seattle DCI | $1,500–$3,500 |
| SPU side sewer permit | Seattle Public Utilities | $2,500–$6,000 |
| Water service connection | Seattle Public Utilities | $3,000–$8,000 |
| Seattle City Light connection | Seattle City Light | $2,000–$5,000 |
| Total permit & utility connection fees | $20,000–$50,000+ |
Why this matters for per-sqft comparisons: If you receive a bid of "$550/sqft" for a 650 sqft DADU ($357,500 total), ask explicitly whether the $20,000–$50,000 in Seattle DCI permits and utility connection fees are included. If they are excluded, the true all-in cost is $377,500–$407,500, or $580–$627/sqft. Always compare bids on a fully loaded all-in basis. Contact Seattle DCI at (206) 684-8850 to request a current fee estimate for your project valuation.
Get an Accurate Cost Estimate for Your Property
Generic cost-per-square-foot ranges are a starting point. For an accurate estimate based on your lot, your goals, and current material costs, start with our free feasibility study.
Get Your Free Feasibility StudyIncludes property-specific cost estimate. No obligation.
Frequently Asked Questions
Why does a smaller ADU cost more per square foot than a larger one?
Fixed costs don't scale with size. Every ADU needs permitting ($8,000-$15,000), design ($10,000-$20,000), utility connections ($10,000-$25,000), a kitchen, a bathroom, and HVAC — regardless of whether the ADU is 400 or 1,000 square feet. When you divide those fixed costs over fewer square feet, the per-sqft number goes up. A 400 sqft ADU with $50,000 in fixed costs adds $125/sqft before construction even begins. An 800 sqft ADU spreads the same $50,000 over twice the area — just $62.50/sqft.
What is the cheapest type of ADU to build per square foot in Seattle?
When you account for total project costs — design, permitting, construction, utility connections, and finishes — even conversions come in around $400/sqft or more. The existing structure helps reduce scope, but plumbing, electrical, HVAC, insulation, egress, and finish work still add up. Conversions save money through smaller total square footage, not a lower per-sqft rate.
Does the cost per square foot include permitting and design fees?
It depends on the contractor. Some contractors quote cost per square foot for construction only, excluding design, permitting, utility connections, and site work. Others include everything in a single all-in number. Always ask what's included in any $/sqft figure you're quoted. At Seattle ADU Builders, our pricing includes design, permitting, and construction — we give you the full project cost so there are no surprises.
How much does an 800 sqft ADU cost to build in Seattle?
An 800 sqft detached ADU in Seattle typically costs $400,000–$560,000 all-in (design, permitting, site prep, utility connections, and construction). That works out to $500–$700 per square foot. Any quote below $400,000 for a detached ADU almost certainly excludes permit fees ($20K–$50K), site preparation, or utility connections ($15K–$40K) — ask the contractor to itemize every line item. Flat lots with straightforward utility access come in at the lower end; hillside lots with extended utility runs and premium finishes are at the higher end.
Your ADU Cost Starts With Your Lot
Every property is different. Let us analyze yours and give you a number you can actually plan around — not a range from the internet.
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