How much does an ADU cost in Clackamas County? It’s one of the first—and most important—questions homeowners ask when considering building a secondary dwelling on their property. Whether you’re planning a detached guest house, converting a garage, or finishing your basement as a rental unit, Accessory Dwelling Units (ADUs) have become a smart solution for adding space, flexibility, and value to your home.
In Clackamas County, ADUs are growing in popularity for multigenerational living, passive rental income, or simply increasing usable square footage without moving. But what does it really cost to plan, permit, and build an ADU here? The answer depends on several key factors—including your lot, layout, and design preferences.
In this post, we’ll break down:
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Typical ADU construction costs in Clackamas County
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What influences the final price (permits, finishes, size, and more)
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Hidden expenses homeowners often overlook
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Local timelines for design, permitting, and construction
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How to plan an ADU that fits your budget and lifestyle
Let’s take a closer look at what you can expect to spend—and how to make the most of your investment.
Average ADU Costs in Oregon
Building an ADU in Oregon is an investment—but one that can pay off in flexibility, long-term value, and even rental income. On average, ADU construction costs in Oregon range from $100,000 to $250,000 or more, depending on the type of unit, size, materials, and site conditions.
Here’s a quick breakdown of the common ADU types and their typical cost ranges:
Detached ADU (Backyard Cottage or Garage Conversion)
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Cost Range: $150,000 – $300,000+
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This is the most expensive ADU type, as it often requires new foundation work, utilities, and separate systems.
Attached ADU (Addition or Bump-Out)
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Cost Range: $125,000 – $225,000
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Attached ADUs use existing structures, saving on materials and site work, but may still require significant framing and utility work.
Interior Conversion (Basement or Above-Garage Unit)
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Cost Range: $80,000 – $175,000
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These are usually the most affordable option because they repurpose existing square footage. However, you’ll still need proper insulation, moisture control, fire separation, and possibly egress upgrades.
While these numbers provide a general idea, actual costs in Clackamas County often vary based on permitting requirements, access, land slope, sewer connections, and zoning limitations—all of which we’ll explore in the next section.
What Impacts ADU Costs in Clackamas County?
In Clackamas County, ADU costs are shaped by more than just square footage. Local zoning, utility access, site layout, and even neighborhood-specific regulations can influence how much you’ll spend on your build. Here are the most common cost drivers for ADUs in this area:
1. Site Conditions & Accessibility
If your lot is sloped, hard to access with equipment, or has limited space between structures, your ADU will likely cost more. Grading, excavation, and foundation work are often more complex in rural areas of Clackamas County.
2. ADU Type
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Detached ADUs (like a backyard cottage or garage unit) usually cost the most due to the need for new utilities, foundation, and sometimes a separate address.
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Attached or internal conversions (like basement ADUs in Oregon City or West Linn) can be more affordable but may still require waterproofing, fire separation, and egress windows.
3. Design & Materials
Your choices for finishes, windows, cabinetry, flooring, and fixtures all affect cost. A simple, rental-focused ADU may use budget-friendly materials, while a luxury in-law suite with high-end finishes can double your final cost.
4. Utility Connections
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Sewer vs. septic: In rural areas of Clackamas County like parts of Molalla or Estacada, you may need to expand or upgrade your septic system, which adds thousands to your budget.
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Water, power, and gas lines must be extended to the new unit. Trenching, metering, and connection fees can vary widely.
5. Local Zoning & SDC Fees
Some cities in Clackamas County waive System Development Charges (SDCs) for ADUs under certain conditions, while others don’t. Knowing your city’s current policy—especially in Oregon City, Happy Valley, or Lake Oswego—can impact your budget by $10,000 or more.
6. Size & Layout
The bigger the ADU, the more it costs—but layout efficiency matters too. For example, plumbing walls that serve both a bathroom and kitchen reduce labor and material costs. Keeping it compact but functional is often the best value.
In the next section, we’ll cover permitting and soft costs specific to Clackamas County so you know what to budget beyond construction.
Typical Permitting & Soft Costs in Clackamas County
When planning a new ADU in Clackamas County, it’s important to budget for more than just construction. Soft costs—which include permits, design work, engineering, and consulting—can add 15–25% to your total project cost.
Here’s what you should expect:
1. Permits & Plan Review Fees
Each city within Clackamas County sets its own permit fees based on size, scope, and location. Here are some rough estimates for ADU permits:
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Building Permit: $1,500 – $4,000
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Mechanical, Electrical, Plumbing Permits: $500 – $2,000
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Plan Review Fees: Usually 65–70% of total permit cost
For example, Happy Valley or Oregon City may charge different fees than unincorporated areas. Some jurisdictions also require public works or planning review for detached ADUs.
2. System Development Charges (SDCs)
These one-time fees are charged by cities or service districts to account for added strain on infrastructure (water, sewer, roads, etc.). Depending on your location:
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SDCs may be waived for ADUs under a certain size (common in Oregon City and parts of West Linn)
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In some areas, SDCs range from $6,000 to $18,000, depending on your utility providers
It’s crucial to confirm current SDC policies with your local jurisdiction before starting design work.
3. Design, Drafting & Engineering
Even a modest ADU requires accurate plans and structural documents for permitting and construction. Costs vary by project complexity:
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Design & Drafting Services: $3,000 – $10,000+
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Structural Engineering (if required): $1,000 – $3,500
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Survey or Site Plan (if needed): $800 – $2,000
4. Additional Soft Costs
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Energy compliance reports (often required): $200–$500
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Arborist reports or soil tests (for steep or forested lots): varies
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Land use application fees (for conditional ADUs): varies by city
These soft costs can sneak up if you’re not prepared—but they’re essential for getting your ADU approved and built the right way. In the next section, we’ll highlight some hidden and often overlooked costs that can catch homeowners off guard.
Hidden or Often Overlooked Costs
Even with detailed plans and a solid contractor, many homeowners are surprised by expenses that fall outside typical construction and permitting. These “hidden” costs aren’t always listed in an upfront estimate—but they can impact your timeline and budget.
Here are the most common ones to watch for in Clackamas County:
1. Utility Trenching & Hookups
If your ADU is detached, you may need new trenches for water, sewer, electricity, and gas.
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Trenching costs depend on the distance, soil conditions, and obstacles like trees, driveways, or existing structures.
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In rural parts of Clackamas County, rocky or sloped terrain can significantly increase this expense.
Typical cost range: $3,000 – $12,000+
2. Fire Separation & Soundproofing
Attached or basement ADUs often require fire-rated drywall, door upgrades, or soundproofing to meet residential code.
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This is especially important in multigenerational homes or rental situations.
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These systems aren’t expensive individually, but they add up in labor and material costs.
3. Separate Electrical or Water Metering
If you plan to rent your ADU, you may want (or be required) to install separate utility meters.
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Not required in most Clackamas cities, but preferred for tenant fairness and management.
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Can add $1,000–$3,500+ depending on the utility provider.
4. Driveway or Pathway Upgrades
You may need to improve access to the ADU, especially for detached units.
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Cities like Lake Oswego or Happy Valley may require legal access paths, parking, or setbacks from property lines.
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Adding gravel or paving may be needed to meet these standards.
5. Landscaping & Lot Restoration
Construction can damage landscaping, tree roots, or irrigation systems.
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In some cities, tree protection measures must be followed, and removal may require fees or mitigation.
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You may need to grade, reseed, or rebuild parts of your yard post-construction.
6. Post-Construction Cleaning & Setup
Not always included in a builder’s contract, final cleaning, furniture staging, or appliance installation may fall on you.
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Plan for at least $500–$1,500 depending on the size of your unit.
Now that you know what to look out for, let’s talk about how long it really takes to design and build an ADU in Clackamas County.
How Long Does It Take to Build an ADU in Clackamas County?
Building an ADU isn’t an overnight process—but with proper planning, you can avoid delays and move forward with confidence. The timeline to build an ADU in Clackamas County depends on your property, your team, and the type of ADU you’re building.
Here’s a breakdown of the major phases and what you can expect:
1. Pre-Design & Research: 2–4 Weeks
This phase includes:
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Deciding what type of ADU fits your lot and budget
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Researching local zoning and setback rules
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Gathering inspiration, wish lists, and general cost info
Clackamas Tip: Some cities like Oregon City or Lake Oswego may offer free or low-cost consultations with planning staff to answer early questions.
2. Design & Permitting: 2–6 Months
This is where your project takes shape.
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Site plan, floor plan, and exterior elevations are developed
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You’ll work with a designer, architect, or design-build contractor
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Permits are submitted and reviewed by the city or county
Common hold-ups in Clackamas County:
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Long review times in peak construction seasons (spring/summer)
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Revisions required for code compliance or site constraints
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Additional reviews for detached ADUs, steep lots, or tree removal
3. Construction: 4–7 Months
Once permits are in hand, construction can begin.
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Interior conversions or attached ADUs may take 4–5 months
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Detached ADUs with foundation work and utility trenching can take 6–7 months or longer
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Inspections, weather delays, and subcontractor availability can all affect the timeline
Clackamas Consideration: In rural or unincorporated areas, utility coordination and site prep may add time to the schedule.
4. Final Inspections & Move-In: 2–4 Weeks
After construction is complete, you’ll go through:
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Final inspections from the building department
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Touch-ups or punch list items
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Utilities activated, appliances installed, and space cleaned up
Then, your new ADU is ready for move-in, guests, or tenants!
From start to finish, you can expect the full ADU process in Clackamas County to take 8 to 14 months. Working with an experienced local builder can shorten that timeline and help you avoid costly rework or permit delays.
Cost-Saving Tips Without Cutting Corners
Building an ADU is a big investment, but that doesn’t mean you have to overspend. The key is knowing where to invest and where to simplify—without sacrificing quality or long-term value. Here are some smart ways to reduce costs on your ADU in Clackamas County:
1. Use Existing Structures When Possible
If you have a basement, garage, or unused space above your garage, converting it into an ADU will almost always cost less than building from scratch.
Basement and garage conversions often require fewer materials, and you may already have plumbing or electrical nearby.
2. Choose a Simple, Efficient Layout
Complex layouts, angled walls, and extra plumbing lines raise labor costs. A compact, rectangular floor plan with the kitchen and bathroom located close together helps keep mechanical work simple and affordable.
3. Plan Ahead for Permitting
Unexpected permit fees or delays can stretch your timeline—and your budget. Hire a builder who knows the permitting process in cities like Milwaukie, Happy Valley, and Oregon City. They’ll help you avoid costly revisions or zoning headaches.
4. Select Mid-Range Materials That Last
You don’t need high-end finishes to create a beautiful space. In fact, many cost-effective materials offer long-term durability.
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LVP flooring, for example, is water-resistant and budget-friendly
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Prefabricated cabinetry is often more affordable than custom without sacrificing function
5. Work With a Local ADU Builder
Hiring a contractor who regularly works in Clackamas County saves time and money. They’ll know how to:
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Submit permits quickly
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Coordinate with local utility providers
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Avoid costly errors or delays during inspections
Bonus: They’ll also understand local style preferences, which helps your ADU blend in and add value.
Building an ADU is a great way to add space, flexibility, and long-term value to your property. But as you’ve seen, the total ADU cost in Clackamas County depends on many factors—from site conditions and permitting to utility hookups, finishes, and design choices.
Whether you’re planning a rental unit, multigenerational housing, or a backyard guest cottage, understanding the true cost upfront helps you stay on budget and avoid surprises.
At Tegman Custom Homes, we specialize in designing and building ADUs that are functional, code-compliant, and built to last. We guide you through every step—from initial planning and permitting to the final walk-through. Our team has experience working in cities like Oregon City, Milwaukie, Lake Oswego, and Happy Valley, and we know how to navigate local regulations to keep your project moving forward.
Looking for other ways to expand your living space? If an ADU isn’t the right fit for your property, a custom home addition might be. Check out our guide to home additions in Clackamas County to compare costs, layouts, and what to expect when building up or out.
Ready to Build Your ADU in Clackamas County?
Let’s talk about your vision and budget. We’ll help you design a space that fits your goals—and your property.
Call us at 503-989-3992
Serving Clackamas County & the Portland metro area
Contact us today for a free consultation!
