AADU vs DADU in Seattle: Which One Fits Your Lot
Quick answer. An AADU is a second home inside your house, usually a converted basement. A DADU is a separate structure in the yard, usually a backyard cottage. The attached version is generally cheaper and faster because the shell already exists. The detached version costs more, disrupts your daily life less while it is being built, and delivers more privacy and more resale appeal. In practice your house and your yard usually make the call for you.
Once a homeowner decides an accessory dwelling unit is worth exploring, this is the next question, and it deserves more than a shrug. The choice between attached and detached changes your budget, your timeline, what your months of construction feel like, who wants to rent the unit, and how the property reads when you eventually sell it.
Our team has walked a lot of South Seattle basements and a lot of South Seattle backyards, and we can tell you that most properties lean clearly one direction once you look at them honestly. Below is how we think it through with clients. For the citywide framework behind all of this, start with our guide to Seattle ADU rules and what you can build.
Quick Facts: The Two Types
- AADU: a legally permitted unit in the home, per SDCI
- DADU: a legally permitted unit on the property but not within the home
- Both include: living, sleeping, kitchen and bathroom facilities and a lockable entrance door
- Both require: a permit, and neither is legal without one
- Neither requires: extra parking, street improvements, or an MHA contribution
- DADUs are not permitted: on lots in shoreline districts, per SDCI Tip 116B
- Two per lot maximum: in any combination of the two types
The Definitions, Straight From the City
Seattle keeps this simpler than most cities do. On its accessory dwelling unit page, SDCI says a legally permitted unit in the home is called an attached accessory dwelling unit, and a legally permitted unit on the property but not within the home is called a detached accessory dwelling unit. Location is the whole distinction. Everything else in this article follows from that one difference.
Both types are complete, independent homes. Both count toward the two-ADU maximum on a lot, so you could do two attached units, or one of each, or in the configurations state law recognizes, two detached units. And both are subject to your zone's height and location standards, which is why the specific dimensions for your parcel come from SDCI or a designer rather than from us.
AADU vs DADU Compared
| Factor | AADU (attached) | DADU (detached) |
|---|---|---|
| Typical form | Basement conversion, split of an upper floor, or an addition | Backyard cottage, new build or converted garage |
| Relative cost | Usually lower, since foundation, walls, and roof exist | Usually higher, since you build everything from grade up |
| Biggest cost risk | Ceiling height, egress, drainage, and unknowns behind old walls | Site work on slope, utility runs, and tree protection |
| Privacy for both households | Shared walls and floors, shared sound paths | Fully separate structure and entrance |
| Yard impact | Little to none | Meaningful, and rear yard coverage limits apply |
| Living through construction | Disruptive, since the work is inside your home | Easier, since the work stays in the yard |
| Permit path | Construction addition or alteration permit | Construction addition or alteration permit, with a pre-approved standard plan option |
| Fastest permitting route | Custom review of your existing conditions | City standard plans, roughly 2 to 6 weeks in most cases per SDCI |
| Resale read | Appeals to income-minded and multigenerational buyers | Broadest appeal, reads as flexible space to almost everyone |
| Ruled out by | Low ceilings, no separable entrance, structural constraints | Shoreline district lots, tight lots, heavy critical area mapping |
Cost: Where the Money Actually Goes
The general rule holds. An attached unit is usually the cheaper build because the expensive parts of a house, the foundation and the envelope, are already paid for. You are adding a kitchen, a bathroom, separation, egress, and mechanical work inside a shell that exists.
The general rule also breaks more often in our neighborhoods than people expect. A South Seattle basement from 1912 might need underpinning to reach legal ceiling height. It might need a window well and an egress opening cut through a foundation wall. It might need perimeter drainage before anyone talks about drywall, because water is the most common thing we find in old basements on a slope. Any one of those can move an attached conversion into the same range as a modest detached build.
The detached side has its own variables, and on hillside lots they are mostly below grade. Site work, foundation on slope, and running water, sewer, and power out to the structure are where detached budgets grow. We go through the money in far more detail in our piece on the cost to build a DADU in South Seattle. The one thing we will say here is that neither number should come from an article. Get two or three real bids on your actual property. On how to pay for it, that is a lender conversation, and a good one to have early.
If you are trying to decide which direction your property points, we are happy to walk it with you and give you an honest read before you hire anyone. Reach out to The Moose Group and we will take a look.
Privacy Cuts Both Ways
Homeowners usually think about their own privacy. The part that gets underweighted is the tenant's, and it shows up in how easily the unit rents and how long people stay.
A detached cottage gives both households real separation. Separate structure, separate entrance, separate outdoor space, no shared floor assembly transmitting footsteps at midnight. That is why detached units tend to lease at the stronger end of a neighborhood's range and turn over less often.
An attached unit shares structure, and no amount of insulation makes that go away entirely. Seattle's building code does require fire and sound separation between an attached accessory unit and the primary dwelling, so there is a real standard being met, not just a wall. Design choices help a lot too: a separate exterior entrance on a different face of the house, stacking the units' quiet rooms against each other, and keeping the shared circulation minimal. But if privacy is the top priority for you or for the family member who will live there, be honest about that early.
Lot Impact and What the Yard Gives Up
An attached unit costs you almost no yard. That matters more than people admit in a city where a usable backyard is part of why you bought the house.
A detached unit takes yard, and Seattle limits how much of the required rear yard accessory structures can occupy. Tree protection interacts with this as well, and we have seen projects reshaped by a single mature tree that was worth keeping. The good news is that no additional off-street parking is required for either type, so you are not sacrificing yard to a parking stall on top of everything else. The caution we repeat from the city's own guidance is that if your property has an existing required off-street parking space, you generally cannot delete it for the cottage without replacing it elsewhere on the lot.
Two lot conditions can rule out the detached option outright. Shoreline district lots, per the city's detached unit tip sheet, do not allow DADUs. And environmentally critical area mapping, which is widespread on South Seattle's slopes, changes the review substantially. Washington's accessory dwelling unit statute expressly does not apply to lots designated with critical areas or their buffers, so those parcels fall back to local review. Check your mapping before you fall in love with a floor plan.
Living Through the Build
This is the factor clients thank us for raising, because nobody thinks about it until they are in it.
An attached conversion happens inside your home. Crews come through your space, your water and power get shut off at intervals, dust travels, and your daily routine bends around the schedule for months. If you work from home, or have small kids, or have anyone in the house who needs quiet, weigh this heavily.
A detached build happens outside. You lose the yard for a season, you live with noise during work hours, and you have equipment and material staging where the garden used to be. But your kitchen works, your bathroom works, and your life stays mostly intact.
One more scheduling note worth knowing. The city's detached unit tip sheet has cautioned that an intake appointment may be a couple of months out, while a pre-approved standard plan can put a permit in hand in roughly 2 to 6 weeks in most cases. Timelines change, so confirm current lead times with SDCI, but the shape of the tradeoff is stable: standard plans move faster than custom ones.
Resale: How Buyers Actually Read Each One
When we put a South Seattle home with an accessory unit on the market, the buyer reaction differs by type in a consistent way.
A permitted detached cottage is the easiest thing to sell. It reads as optionality. One buyer sees rental income, another sees a studio or an office, another sees a place for an aging parent, another sees a guest house. Nobody has to share their home to get value out of it, so the appeal is wide.
A permitted attached unit sells to a narrower but genuinely motivated group. Buyers who want the income to help with the payment care a lot, and so do families planning to bring a parent or an adult child under the same roof. Buyers who want the whole house to themselves see a basement they will have to un-convert, which is a fair reaction and one you should price for rather than argue with.
In both cases the permitted status is the hinge. SDCI is explicit that these units are not legal unless established through a permit process, and an unpermitted unit is not something we can market as a dwelling unit. We unpack what all of this means for your number in does an ADU add value in South Seattle.
Where Each One Tends to Win in South Seattle
Beacon Hill is attached-unit country on the slope, where daylight basements are common and often already have a separate grade-level entrance, and detached-unit country on the flatter plateau lots. Our deeper look at Beacon Hill ADU and DADU investment gets into the block-by-block picture.
Columbia City rewards proximity to the station and the business district, which favors whichever unit type your lot can actually carry rather than a general preference. Mount Baker has the grade changes and the larger lots, so detached units are feasible on more parcels but site work costs more. Rainier Beach is where we see the most room for a genuine backyard cottage without crowding the main house.
For a fuller neighborhood breakdown, see our South Seattle ADU neighborhood guide, and if renting is the goal, renting out your Seattle ADU covers what comes after the final inspection.
How We Would Decide
Start with what the lot removes from the menu, not with what you prefer. If you are in a shoreline district or your rear yard cannot hold a structure, the decision is made. If your basement has seven-foot ceilings and no path to a separate entrance, the decision is made the other way.
Where both are genuinely possible, we would ask three questions. Who is going to live there, and how much separation do they need. How much yard are you willing to give up permanently. And what does the property need to look like on the day you sell it. A designer or architect will tell you what fits. SDCI will tell you what is allowed. A CPA will tell you how the income and the added assessed value affect your taxes. We will tell you what the finished result does to your position in this market.
Frequently Asked Questions About AADUs and DADUs in Seattle
What is the difference between an AADU and a DADU in Seattle?
The city draws the line by location. Seattle's Department of Construction and Inspections defines a legally permitted unit in the home as an attached accessory dwelling unit, or AADU, and a legally permitted unit on the property but not within the home as a detached accessory dwelling unit, or DADU. A converted basement is the classic AADU. A backyard cottage is the classic DADU. Both are complete homes with a kitchen, a bathroom, sleeping space, and a lockable entrance door, and both require a permit.
Is an AADU cheaper than a DADU in Seattle?
In most cases yes, because you are working inside a foundation, walls, and a roof you already own. A detached unit is new construction and carries its own foundation, its own envelope, and its own utility runs across the yard. The exception is a basement that needs underpinning for ceiling height, egress windows cut into a foundation wall, or major drainage work, all of which are common in South Seattle and can close the gap quickly. Get real bids on both before you assume.
Which adds more privacy, an AADU or a DADU?
A detached unit, and it is not close. A DADU has its own walls on all four sides, its own entrance, and its own patch of outdoor space, so neither household hears the other's floors, plumbing, or late nights. An AADU shares structure with your home, which means shared sound paths and often a shared exterior door area. Good design closes some of that gap, and Seattle's building code requires fire and sound separation between an attached unit and the primary dwelling, but shared walls stay shared walls.
Can you build a DADU on any Seattle lot?
No. Detached units are allowed in specific residential zones, and the city's tip sheet for detached units states that DADUs are not permitted on lots in shoreline districts. Your lot's size, shape, required yards, tree coverage, and any environmentally critical area mapping also constrain whether a second structure fits. Those are parcel-level answers that come from SDCI and from a licensed designer, not from a rule of thumb.
Which one do buyers prefer when you sell?
In our experience across South Seattle, a permitted detached cottage draws the widest interest because it reads as flexible. It can be a rental, a home office, a place for a parent, or a guest space, and it does not require the buyer to share their house. A permitted attached unit appeals strongly to buyers who want the income or who need family close by, and less so to buyers who want the whole house to themselves. The word doing the heavy lifting in both cases is permitted.
Can I convert my garage into an ADU in Seattle?
Conversions of existing structures are contemplated in Washington's accessory dwelling unit statute, RCW 36.70A.681, which requires cities to allow accessory dwelling units to be converted from existing structures including detached garages, even where those structures violate current code requirements for setbacks or lot coverage. That is the statutory framing. Whether your specific garage works depends on its foundation, framing, height, drainage, and whether your lot carries a critical area designation, since the statute does not apply to lots designated with critical areas or their buffers. Have a designer look at it.
Which is more disruptive to live through?
An attached unit, by a wide margin. That work happens inside your house, with your utilities getting shut off, dust in your living space, and crews coming through your front door. A detached build keeps the work in the yard, so you lose your outdoor space and gain construction noise but keep your kitchen and your routine. Families with small children or anyone working from home should weigh this seriously rather than treating it as a footnote.