Your Lot May Already Have an Answer
You ran out of room, and you need more than what a standard addition can give you.
You need a full, separate living space, and that’s where an Accessory Dwelling Unit (ADU) comes in.
But which type best fits your needs?
In this post, you’ll see how attached and detached ADUs stack up on cost, privacy, space, timeline, permitting, and resale value.
By the end, you’ll know which one fits your lot, your budget, and the way you plan to use the space.
What’s the Difference Between an Attached and Detached ADU?
An attached ADU shares at least 1 wall with your main house and is built as an addition. A detached ADU is a freestanding building set somewhere else on your lot.
Here’s a breakdown of their main differences:
| Feature | Attached ADU | Detached ADU |
|---|---|---|
| Starting cost (1 bed, 1 bath) | From $325,000 | From $290,000 |
| Privacy | 1 shared wall, thinner sound buffer | No shared walls, full separation |
| Space needed | Typically 600–900 sq. ft. of new space added onto the house; uses yard, but works on tighter lots | Typically 600–900 sq. ft. freestanding; needs open backyard with room to clear setbacks on all sides |
| Build time | 12 to 16 weeks | 12 to 16 weeks |
| Resale value | Adds value, blends with the home | Adds value, reads as a second home |
Cost: Why Attached Usually Starts Higher
Here is where most guides get it backward when it comes to ADU costs: they say attached is the cheaper route.
It usually isn’t.
A detached new build starts around $290,000 for a 1-bed, 1-bath unit.
An attached one starts higher, near $325,000.
The reason is the work itself.
Tying a new unit into your existing home means demolition, matching the roofline, and reworking shared mechanical systems.
That integration runs more than pouring a clean foundation out in open space.
Per square foot, either type costs more than a brand-new house because you’re packing a full home’s worth of systems into a small footprint.
Those fixed costs do not shrink with the floor plan.
The design elements also affect cost.
Privacy: How Much Separation You Get
Privacy is the sharpest line between them.
A detached unit stands alone.
No shared walls and no sound passing between the two homes.
That separation is why detached tends to win for renters and long-term guests who want their own space.
An attached unit shares at least 1 wall, so the sound buffer is thinner.
What it gives up in separation, it gains in proximity, sitting right against the main house.
That closeness is the whole point when you’re housing a parent or an on-site caregiver who needs to be nearby.
Space: Where Each One Can Sit on Your Lot
Both a new attached and a new detached unit add square footage, so both take up some yard space.
The real difference is placement.
A detached unit is its own building, so it has to clear setbacks on every side and, in many jurisdictions, sit behind the main house.
That means you need a stretch of open backyard with room all the way around it.
An attached unit grows off the wall of your home, so it only has to clear setbacks on its open sides.
On a small, narrow, or steep lot where a freestanding building can’t meet those clearances, that’s often what makes attached the buildable option.
Before THE Remodel Group designs anything, we run a full feasibility study, checking lot lines, setbacks, zoning, permit history, and HOA rules so the plan you fall for is one we can actually build.
Build Timeline: When You Get the Keys
On timeline, the two basically tie.
A new attached unit and a new detached unit each finish in 12 to 16 weeks once we break ground.
Permitting: What the City Requires of Each
Attached or detached, every ADU clears the same gate.
An ADU is a self-contained home with its own kitchen, bath, and separate entrance, which is exactly why the permitting carries real weight.
If all you really want is a home office or a home gym, that permitting and those separate utilities make an ADU the wrong tool for the job.
Jurisdiction is the first part that sets the number of ADUs you can have.
Most allow 1 ADU, some allow 2, and lot size only matters after that.
Even on a large lot, FAR limits on total coverage can rule out an ADU that would otherwise be permitted.
Size caps typically land between 600 and 900 square feet, set by your jurisdiction, the size of your existing home, the type of ADU you build, and whether it has to be fully ADA compliant.
The difference shows up in placement.
A detached unit has to satisfy extra rules about where it can sit on the lot, while an attached unit follows the lines of the home it’s built onto (though they are still subject to setbacks).
ADU rules vary by city across the Portland metro.
We confirm what your jurisdiction allows before we draw a single line.
Resale Value: What Each Adds When You Sell
Both types add value; they just add it differently.
A detached unit reads as a second home on the property, which appeals to buyers picturing rental income or a separate space for family.
An attached unit blends into the main house, appealing to those who want family members to use it.
Either way, the value holds best when the design works for the long haul.
Our team carries Universal Design certification, so wider doorways, zero-threshold entries, accessible bathrooms, and single-level living go in from the start.
They can lift property value by 20 to 35% in high-demand neighborhoods.
We design to the widest range of future occupants, because that is what protects your money the day you sell.
A Third Option: Converting Space You Already Have
If you’d rather not give up any yard, there’s a third path.
We also convert an existing garage, basement, or attic into a permitted ADU, starting around $295,000.
Just know the savings are thinner than they look.
Garages were never built to housing code, so insulation, wiring, ceiling height, and structural fixes all have to come up to standard, and that work eats into whatever you hoped to save.
Conversions also run longer, 16 to 20 weeks, since we’re reworking what’s already there instead of building clean.
But when your lot is tight, and the existing bones are good, it turns dead space into a home someone can actually live in.
How to Choose Between The 3
When choosing the right ADU for you, start with how you’ll actually use the space.
- Want the unit to stand to be fully separate from the main residence? Detached gives you that separation, with its own walls and a slice of yard that belongs only to the unit.
- Housing a parent or an on-site caregiver? Attached keeps them a few steps from the main house instead of across the yard, with no second structure to build.
- Trying to keep your whole backyard? A garage or basement conversion adds the unit inside space you already have.
Then there’s the lot itself.
If your yard has development constraints, such as because it’s near a floodplain or environmental overlay along a creek, setbacks may be impacted.
If your yard is small or steep, the setbacks may rule out a freestanding build, making attached or a conversion your real options.
And one honest caution: an ADU is a complete dwelling unit, so if all you really want is a home office or a home gym, the permitting and the separate utilities make it the wrong tool for the job.
Build one when someone is going to live there.
Budget counts, too, but it’s rarely the whole story.
Our job is to guide you to the right fit, not the biggest invoice, and sometimes the smarter move is a tighter layout or a different material instead of a bigger check.
Build the Right Space for Your Family with THE Remodel Group
We design and build attached units, detached units, and conversions across the Portland metro.
Want to know what your lot can hold and what it will cost?
Fill out our contact form today or give us a call, and we will run your project through our Predictive Pricing tool so you get an honest cost range before you ever commit to design.
