Attached ADU vs. detached ADU (DADU): which is right for your property?

Once you have decided to add an accessory dwelling unit, one question shapes everything that follows: build it attached to the house, or detached in the yard.

An attached ADU, sometimes called an AADU, is carved out of or added onto the existing home. A basement converted with its own entrance and kitchen, a unit above an attached garage, an addition off the back with a separate door. A detached ADU, or DADU, is a standalone building in the yard. A backyard cottage, or a unit over a detached garage.

Most guides answer this with zoning tables, which vary by city and change with every ordinance. The more useful comparison is the one nobody publishes: what each one costs you, what it does to your privacy, and what it does to your life while it is being built.

What you are actually paying for

Neither type has a fixed price, but they differ in what you buy, and that is what drives the gap.

An attached ADU reuses structure you already own. The foundation exists. The roof exists. Two or three of the walls exist, along with the exterior cladding. Utilities already run to the building. What you are paying for is interior reconfiguration, a code-compliant separate entrance, egress that meets requirements for a bedroom, sound and fire separation between units, a new kitchen and bath, and usually electrical and plumbing capacity upgrades.

A detached ADU is a small house. You pay for excavation and a new foundation, a complete structural shell, a roof, an insulated and clad envelope, windows and doors, and then all the same interior work an attached unit needs. On top of that come the costs an attached unit avoids entirely: trenching water, sewer, and power across the yard, site work and drainage, and restoring the landscaping you dug up.

Permit costs follow the same pattern. As a planning allowance, an attached ADU typically runs about 2 to 4 percent of project cost in permits, while a detached ADU runs about 2.5 to 5 percent, reflecting the extra site, utility, and structural review a standalone building requires.

There is one honest caveat on the attached side. Conversions look cheap until you open the walls. An older basement may need underpinning for ceiling height, waterproofing, egress windows cut into a foundation wall, and a full electrical service upgrade. Any of those can close the gap with new construction quickly, which is why the feasibility review matters more for conversions than for new builds.

Across both types, a finished unit in this market generally lands somewhere between about $300,000 and $700,000 or more, with roughly 6 to 12 months of construction.

Privacy: the factor that decides most rentals

This is where detached wins decisively, and it is usually the real reason people choose it.

A detached cottage has its own walls, its own roof, its own entrance, and its own outdoor space. There is no shared wall transmitting a television through the house at midnight, no shared entry path, and no sense that someone is living inside your home. Tenants feel it too, which is why detached units generally command stronger rents and rent faster.

An attached ADU can be made private, but it takes deliberate design: a genuinely separate exterior entrance rather than a shared hallway, serious sound separation between the units, and ideally a sightline arrangement that keeps the two households from meeting every time they take out the bins. Done well, it works. Done as an afterthought, you feel it every day.

For housing a family member, this cuts both ways. An attached unit keeps an aging parent close enough to check on easily, which is often exactly the point. A detached unit gives an adult child independence. Neither is better, they serve different relationships.

Construction disruption

An underrated difference, and one that matters enormously if you are living in the house.

Building detached is remarkably easy to live through. The work happens in the yard. Your kitchen works, your bathrooms work, you keep your routine. You lose the yard and gain construction noise, but your home remains a home.

Building attached means construction inside your house. Demolition, dust, trades moving through your living space, and often a period with a utility off. If the attached ADU involves your basement or garage, you lose that space entirely during the build, along with whatever is stored there.

So if you have young children, work from home, or simply cannot face months of interior disruption, that consideration alone often settles the question.

What your lot needs to have

The decision is often made for you by what the property physically offers.

A detached ADU needs yard. Enough buildable area after setbacks, enough separation from the main house, and a route for utilities. It also needs construction access. A lot with no alley and a narrow side yard means materials, excavators, and concrete have to reach the back some other way, which adds cost and sometimes rules the option out entirely. Trees, slopes, and critical areas take further bites out of the buildable envelope.

An attached ADU needs usable interior space. A basement with adequate ceiling height, a garage that can be converted, a bonus room over a garage, or a wall where an addition can reasonably attach. It also needs somewhere to put a genuinely separate entrance, which is harder than it sounds on a house designed around one front door.

A simple test: walk your property and ask which you have more of, unused indoor space or unused outdoor space. That answer is right more often than any spreadsheet.

Rental income and resale

Rental. Detached units generally rent for more and to a wider pool of tenants, because they read as a house rather than an apartment in someone's basement. Attached units still rent well, particularly with a private entrance and good sound separation, and they often make more sense for a lower rent point or for family.

Resale. Both add a second legal dwelling, which is the valuable part. A detached cottage tends to be the more visible, more marketable asset, since a buyer can see a whole separate building. An attached unit's value depends heavily on how convincingly separate it feels. A basement unit with its own entrance, good light, and real sound isolation is a genuine second home. One that shares a stairwell with the main house is a room with a kitchen in it.

Financing and appraisal treat these similarly in most cases, but the specifics vary by lender and market, so that is a conversation for your lender rather than your builder.

Which one suits which situation

Then check your city's rules

Everything above is about your property and your goals. The other half of the decision is what your jurisdiction allows, and that genuinely varies. Some cities permit two ADUs per lot, some cap sizes differently for attached and detached units, and height and setback standards differ meaningfully between neighbouring cities.

We cover the specifics in our city guides: building an ADU in Issaquah and Seattle's pre-approved DADU plans. Confirm the rules for your address before you commit to a type, because in some cases the code makes the decision for you.

The lot usually answers the question

Strip away the preferences and this decision is mostly practical. If you have a usable basement or garage and a tight budget, attached is the efficient answer. If you have yard, access, and privacy is the point, detached is worth the premium. If you have both, the deciding factor is usually who will live there and whether you can tolerate months of construction inside your home.

What you should not do is choose the type first and then try to make the property accommodate it. Read the lot, confirm your city's rules, then pick.

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