Neil Kelly designs and builds attached and detached ADUs across Portland, Seattle, Eugene, and Bend. Fixed-price contracts, in-house craftspeople, and a process designed for the complexity of adding livable space to an existing property.
Attached and detached ADUs are different projects with different starting points. Here's how to figure out where yours fits.
An attached ADU lives inside or connects directly to your existing home; a finished basement, a converted garage, or a new addition built specifically for the purpose. Where you start depends on what your home already gave you to work with.
under the three levels for this block:
The Space is Mostly There.
Your basement or bonus room already functions close to independently. It has usable ceiling height, plumbing nearby, and a workable layout, common in Portland's post-war additions and finished lower levels. The project is about bringing it up to code: dedicated utilities, a separate entry, insulation, and the finishes that make it a real ADU. Typical scope: Targeted finish work, utility separation, and dedicated entry.
The Bones are There. The Space Isn't.
You've got an unfinished basement, detached garage, or other raw space, often original to a Portland home built decades ago, with good structural bones but nothing finished. The project means building it out from the studs: framing, plumbing, electrical, HVAC, and finishes, into a fully functioning independent unit. Typical scope: Full conversion, systems installation, and finish build-out to meet ADU code requirements.
The Space Doesn't Exist Yet.
You want an attached ADU, but your existing home doesn't have a space worth converting. That means new construction: an addition connected to your home, designed and permitted through the city as an additional dwelling unit. Typical scope: Addition design and build, full systems, independent utilities.
A detached ADU is a standalone structure on your property, purpose-built and fully independent from the ground up. There aren't scope levels here the way there are for a conversion. Every detached ADU starts from the same point: site assessment, design, permitting, foundation, framing, systems, and finishes. Size and placement potential will change from lot to lot. Neil Kelly designs and builds detached ADUs across Portland to the same standard as our custom homes: fixed-price contract, in-house craftspeople, five-year warranty.
Whether you're converting existing space or building from the ground up, most Neil Kelly projects move through six phases. ADU projects involve a distinct set of requirements, utility considerations, and code thresholds. Our process is designed to handle that complexity so you can move forward with confidence at every stage.
A Neil Kelly designer will come to your property for a free 60-to-90-minute conversation. We'll assess the site or existing space, talk through what type of ADU makes sense for your property, and give you an honest read on scope, timeline, and budget. No commitment required.
Floor plan options, site or space configuration, materials direction, scope confirmation. For detached ADUs, we'll look at how the unit sits on your lot. For attached, we'll work through how the space converts or connects. You'll see alternatives before any detailed drawings begin.
Full plans, full specs, and every material and system selected. This is where your ADU becomes a complete, code-compliant set of construction documents, including the utility separation and egress requirements specific to ADUs.
The number you see is the number you pay. Permits filed, materials ordered, schedule set. ADU permitting involves more steps than a standard remodel. Our fixed-price contract makes sure that complexity is accounted for before work begins.
Built by Neil Kelly's in-house craftspeople. A dedicated project manager runs the site, coordinates every trade, and keeps you updated weekly. You'll always know where your home stands.
Final walkthrough of every room, feature, and system in the completed unit. Anything that's not right gets fixed before you sign off. Five-year warranty coverage starts on completion.
These five factors will have the biggest impact on what your ADU actually costs. Understanding them before you meet with a designer will give you more clarity and confidence for your first conversation.
Whether you're converting an existing space or building a new structure on your property is the single biggest cost variable in an ADU project. A basement with nearby plumbing and adequate ceiling height costs less to finish than a bare-bones garage. A compact detached ADU costs less than a larger one with a complex roofline. Where you start and what you're building toward sets the baseline for everything else.
An ADU is a complete home, which means material decisions stack up across every room and system., and every selection sits on a spectrum from durable and budget-conscious to fully custom. A designer can help you create a finish strategy that fits the way the unit will be used, whether that's long-term family housing or a rental.
For detached ADUs, lot conditions shape what building there will cost: slope, soil stability, drainage, and utility connection distance all vary by property. For attached ADUs, what's already in the space determines how much work it takes to get it to code. We evaluate carefully before pricing so neither comes as a surprise.
Taxes and labor costs vary across Bend, Eugene, Portland, and Seattle, as do permitting fees and inspection requirements.
ADU projects in the Pacific Northwest come with a predictable set of complications. Here's what we encounter on projects like this and how we handle it.
ADU permitting is more involved than a standard remodel permit, and in Portland and Seattle the process takes longer than most people plan for. Some jurisdictions have streamlined ADU approvals in recent years; others haven't. We know the landscape in each of our markets, file early, and build a timeline into your contract that reflects the actual process.
Basements and detached structures are two of the most moisture-vulnerable building types in the PNW. After 80 years working in this climate, we know how to build against it. Every ADU we deliver includes the venting, vapor barriers, moisture-rated materials, and detailing that keep a finished unit performing the way it should year after year
Attached ADUs often require opening up parts of a home that haven't been touched in decades. Outdated wiring, undersized panels, aging plumbing, and worn framing are common finds in PNW homes built 50–100 years ago. We plan for what we're likely to find and communicate quickly when something affects the scope or budget.
Converting a basement or connecting a new structure to an existing home puts demands on existing framing and foundations that weren't part of the original design. We assess structural capacity early so what we find becomes part of the plan before construction begins.
In Portland, the maximum size for an ADU is 75% of the living area of the primary structure or 800 sq ft, whichever is smaller. However, if the ADU is in the basement of a primary home that is five years old or more, this limit does not apply. Detached ADUs must be set back 40 feet from the front lot line or behind the rear wall of the primary structure. The side and rear setbacks are zone specific.
Eugene's maximum ADU size is also 800 sq ft. It must be further from the front lot line than the parimary structure, and set back at least 4 ft from the side and read property lines.
In Bend, the maximum ADU size is 800 sq ft as well. The front setback in most residential districts is 10ft, and 4 ft from the side and rear.
Washington ADU regulations vary by city. In Seattle, property owners can have two ADUs, but at least one has to be attached. Detached ADUs can be 1,000 sq ft at maximum, and in most cases, must be set back at least 20 ft from the front lot line and 5 ft from the front and rear.
In Bellevue, ADUs can be 1,200 sq ft or 40% of the primary structure, whichever is larger. They have the same front and side setback requirement as the primary structure, with a 5 ft. setback from the rear property line.
Yes. ADU-specific loans from lenders like RenoFi and others are available. These often allow borrowing based on the after-renovation value of your home.
Rental income varies significantly by market, ADU size, and amenities. As of 2026:
In Portland, an ADU can usually generate $1,000-$2,500 per month.
In Eugene, ADU owners can expect to make $1,000-$2,000 per month
ADUs in Bend can generate between $2,000 and $2,400 per month
And in Seattle, an ADU can typically rent for $1,600-$3,200 per month
Ready to talk about your ADU plans? Connect with the Neil Kelly team in your area. Each location has its own designers, project managers, and design center.
Talk with a Neil Kelly designer about what an ADU could look like on your property. They'll evaluate the site or space, walk through the realistic options, and help you figure out where to start.
