Quick Answer
Portland ADUs: $180K–$350K
Permits: $3K–$11K · Updated April 2026
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Quick Cost Overview by Type
| Type | Size (sq ft) | Cost Range | Cost / Sq Ft | Timeline |
|---|---|---|---|---|
| Basement ConversionMost Affordable | 400-600 | $75k - $150k | $125 - $250 | 3-5 months |
| Garage Conversion | 350-500 | $90k - $150k | $150 - $300 | 3-6 months |
| Small Detached ADU | 400-600 | $150k - $240k | $250 - $400 | 5-8 months |
| Mid-Size Detached ADUMost Popular | 600-800 | $180k - $280k | $250 - $350 | 6-9 months |
| Large Detached ADU | 800-1000 | $220k - $350k | $250 - $350 | 7-10 months |
| Two-Story ADU | 1000-1200 | $300k - $450k | $300 - $375 | 8-12 months |
| Prefab/Modular ADUFastest Timeline | 400-800 | $180k - $300k | $350 - $450 | 4-7 months |
* Estimates reflect 2026 market rates in the Portland Metro area. Actual costs vary by site conditions and finish quality.
ADU Cost Per Square Foot in Portland
| Finish Level | Cost per Sq Ft |
|---|---|
| Standard Finish | $200 – $250 / sq ft |
| Mid-Range Finish | $250 – $320 / sq ft |
| High-End Finish | $320 – $400 / sq ft |
Use the free cost estimator below to calculate your total based on your specific size and finish level.
Complete Cost Breakdown (2026)
Phase 1: Soft Costs (Pre-Construction)
Portland Advantage
Portland often waives System Development Charges (SDCs) for ADUs if they are not used as short-term rentals for 10 years, potentially saving you $15k-$20k.
Phase 2: Site Work & Utilities
Portland Considerations
Excavation costs can spike in hilly areas like the West Hills. Additionally, older neighborhoods may require sewer line upgrades to handle the extra load.
Phase 3: Foundation & Structure
Construction Note
Stick-framing is standard in Portland. While slab-on-grade is cheaper ($12k+), many sloped lots require raised foundations ($20k+) or specialized stem walls.
Phase 4: Exterior Envelope
Climate Considerations
Portland's rainy climate demands high-quality roofing, flashing, and moisture barriers. Don't skimp on waterproofing or windows.
Phase 5: Systems & Mechanicals
Energy Efficiency
Mini-split heat pumps are the gold standard for Portland ADUs—providing both heating and cooling efficiently year-round.
Phase 6: Interior Finishes
Style Notes
Portland ADUs often favor a 'Northwest Modern' aesthetic—clean lines, natural wood accents, and durable LVP flooring.
Phase 7: Green Building & Fees
Oregon Code
New Oregon Residential Energy Codes require strict efficiency standards. Budget for better insulation and high-performance windows.
Landscaping
$4k - $12k
Pathways, fencing, planting
Stormwater Mgmt
$1.5k - $5k
Drywells, rain gardens
Contingency
10% - 15%
Always budget for surprises
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Neighborhood Cost Variations
Construction costs in Portland aren't uniform. Labor rates, site accessibility, soil conditions (especially in the hills), and local demand can swing prices by 10-20% depending on your zip code.
Premium Areas
$350 - $450 / sq ft
Neighborhoods
NW Portland, SW Hills, Inner SE (Ladd's Addition), Lake Oswego, West Linn
Cost Drivers
- Steep slopes requiring complex engineering
- Difficult site access for heavy machinery
- Historic district design requirements
Mid-Range Areas
$250 - $350 / sq ft
Neighborhoods
NE Portland, Inner Eastside, North Portland (St. Johns), Beaverton, Tigard
Cost Drivers
- Standard flat lots with good alley access
- Typical soil conditions
- Competitive contractor availability
Value Areas
$200 - $300 / sq ft
Neighborhoods
Outer East Portland, Outer North, Gresham, Milwaukie
Cost Drivers
- Larger lots with easier construction access
- Simpler permitting in some outer zones
- Lower localized labor/subcontractor rates
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ADU Cost in Clackamas County — What You'll Pay in 2026
Clackamas County ADU costs range from $160,000 to $320,000 depending on whether you're building in unincorporated areas, Milwaukie, Lake Oswego, or Oregon City. County permits are typically $4,000–$8,000, and the approval process takes 8–12 weeks. Detached ADUs in Clackamas County average $200–$250 per square foot — about 10–15% less than Portland proper.
While Clackamas County is part of the Portland metro area, it has its own distinct permitting process, zoning regulations, and fee structures. Building an ADU in Clackamas County is often slightly more affordable than within Portland city limits, particularly due to lower System Development Charges (SDCs).
Cost Comparison: Clackamas County vs. Portland
| ADU Type | Clackamas County | Portland City Limits |
|---|---|---|
| Garage Conversion | $80,000 - $130,000 | $90,000 - $150,000 |
| Detached ADU (800 sq ft) | $160,000 - $250,000 | $180,000 - $280,000 |
| Average Permit Fees | $4,000 - $8,000 | $6,000 - $12,000 |
System Development Charges (SDCs)
Clackamas County generally has lower SDCs compared to Portland. While Portland offers SDC waivers if you agree not to use the ADU as a short-term rental for 10 years, Clackamas County's baseline fees are often lower outright, though exact fees depend heavily on your specific water and sewer district.
- Sewer SDCs vary significantly by provider (e.g., Water Environment Services).
- Transportation SDCs may be assessed based on the specific location.
Rural vs. Urban Differences
Clackamas County has both urban zones (inside the Urban Growth Boundary) and rural zones. In rural areas, you must often account for well water flow tests and septic system evaluations. If your existing septic system cannot handle the additional capacity of an ADU, upgrading it can add $15,000 - $25,000 to your overall project cost.
Clackamas County ADU FAQ
Portland ADU Permitting Timeline
Pre-Application Conference
An optional but highly recommended meeting with city staff to discuss your proposal, zoning, and potential issues early on.
Prepare / Submit
Preliminary site plan, basic floor plan sketches.
Portland Pro Tip
"Mandatory for projects valued over a certain threshold, but do it anyway to avoid surprises."
Design & Plans
Your architect or designer creates the full set of construction documents required for the permit application.
Prepare / Submit
Site plan, floor plans, elevations, structural calcs, energy compliance forms.
Portland Pro Tip
"Hire a designer familiar with Portland's specific 'Residential Infill Project' rules."
Permit Application
Submitting your package to the Bureau of Development Services (BDS). The intake review begins.
Prepare / Submit
Completed permit application forms, plan sets, and intake fees.
Portland Pro Tip
"Use the Development Hub PDX online portal for faster processing and tracking."
Plan Review & Corrections
City planners review your plans. 'Checksheets' (correction lists) are common and normal.
Prepare / Submit
Revised plans addressing every specific comment from the city.
Portland Pro Tip
"Respond to checksheets quickly. The clock stops on their side until you reply."
Permit Issuance
Once approved, you pay the remaining fees, and the permit is issued. You're ready to build!
Prepare / Submit
Final permit fees (often $2k-$5k depending on valuation).
Portland Pro Tip
"Check if you qualify for the SDC waiver—ensure you sign the covenant if required."
Construction & Inspections
Building phase with required inspections at foundation, framing, mechanicals, and final.
Prepare / Submit
Approved plans on-site at all times for inspectors.
Portland Pro Tip
"Schedule inspections 2-3 days in advance during busy summer months."
Portland ADU Requirements Checklist
Zoning & Size Limits
- Max Size: Up to 800 sq ft (or 75% of main house size, whichever is less). Some lots allow up to 1,000 sq ft.
- Height: Generally match the main house or up to 20-25 ft for detached ADUs.
- Setbacks: Usually 5 ft from side and rear property lines (converted garages may keep existing setbacks).
- Occupancy: No owner-occupancy requirement for either unit (great for investors!).
Units & Parking
- Allowance: Most residential lots allow 1 ADU. Some larger lots or corner lots may allow 2 (detached + internal).
- NO PARKING REQUIREDPortland does not require you to add extra parking spots for an ADU. You can even convert your garage without replacing the lost spot.
- Design: Exterior finish must generally match or complement the main house.
Permit Fee Breakdown
Typical 2026 fees for a 600–800 sq ft detached ADU. Always verify with City of Portland Bureau of Development Services before submitting.
| Fee | Typical Range |
|---|---|
| Plan check | $1,500–$3,500 |
| Building permit | $2,000–$4,500 |
| School fees | Not assessed (Portland exempts ADUs) |
| Impact / capacity fees | $0–$3,500 (SDC waiver for ADUs <800sf, expires 2026) |
| Total typical range | $4,300–$11,500 |
Climate & Construction Considerations
Portland's wet, mild climate (Pacific maritime) drives detailing more than insulation: rain screens, flashing, and proper grading are non-negotiable to avoid moisture intrusion. Average rainfall is 36" with 156 rainy days/year. Foundation drainage and a continuous capillary break are required by code. Energy code (Oregon Reach Code) demands R-21 walls, R-49 roofs, and heat-recovery ventilation for tight envelopes.
Local Code & Zoning Notes
Portland allows 800sf detached ADUs by-right citywide; up to 75% of primary or 1,000sf (whichever is smaller) with a 4ft setback. The SDC waiver for ADUs under 800sf expires in 2026 — file before then to save $7K–$12K. Tree removal triggers a separate Urban Forestry permit. Short-term rental requires a Type A or Type B accessory rental permit; owner occupancy is required for STR.
Frequently Asked Questions
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