Most people do not start an ADU conversation because they love permits. They start because the house needs to do more. Maybe your parents need a private place without moving across town. Maybe the basement can finally become legal living space. Maybe the garage is wasting half the lot. Maybe the backyard is big enough for a cottage, but you are not sure if Seattle will actually let you build it.
That is the real starting point for adu plans seattle and dadu plans seattle. Not a pretty drawing first. Not a square-foot number from the internet. The first real question is: what can this lot, this house, this sewer line, this ceiling height, this access, and this permit path actually support?
Renova Contractors LLC helps homeowners coordinate plans, permit drawings and buildability for ADUs, DADUs, basement ADUs, attached ADUs and backyard cottages. We are a remodeling contractor, not an architecture firm, so we are clear about the roles. When the project needs a licensed architect, designer or structural engineer, we coordinate with the right professional and tie the drawings back to a construction scope. If you want the broader planning service, see our architectural plans and permit coordination in Seattle.
This article is for homeowners searching for seattle adu architect, adu architect seattle, dadu architect seattle, seattle dadu architects, backyard cottage plans seattle, adu permit drawings, dadu design build seattle, or adu design build seattle and trying to figure out what actually happens before a permit gets approved.
ADU, AADU, DADU: Same Family, Different Problems
Seattle uses specific language here. SDCI says: "An accessory dwelling unit (ADU) is a separate living space within a house or on the same property as an existing or proposed house." SDCI also explains that a legally permitted unit in the home is called an attached accessory dwelling unit, or AADU, and a legally permitted unit on the property but not within the home is called a detached accessory dwelling unit, or DADU.
That wording sounds dry, but it changes the project fast.
An attached ADU usually means the existing house is doing a lot of the decision-making. It may be a basement ADU, lower-level studio, garage conversion, or attached addition. You are dealing with ceiling height, egress, stairs, plumbing routes, old electrical, moisture, framing, smoke and carbon monoxide alarms, utility access, and how the new unit enters and exits.
A DADU is a different animal. A backyard cottage has its own structure, so the site plan becomes the main story. You have to think about lot lines, setbacks, height, alley access, side sewer, drainage, foundation, utilities, energy code, mechanical ventilation, construction access, tree conditions, and whether a retaining wall or stairs are quietly waiting to become a major line item.
Seattle's ADU ordinance says: "No lot may have more than two accessory dwelling units." It also says accessory dwelling units may be "attached, detached, or stacked" and must be located on the same lot as the principal dwelling unit. For size, the ordinance says the gross floor area of an accessory dwelling unit with up to two bedrooms may not exceed 1,000 square feet, and with three or more bedrooms may not exceed 1,200 square feet, except for listed exceptions.
So yes, the floor plan matters. But it is not the first thing that matters.
The Past Project Lesson: Permitted Does Not Always Mean Build-Ready
One of our past ADU/DADU project reviews was on Capitol Hill, near Lakeview Boulevard. The package already had plans and permit information. The scope included a lower-level studio unit addition to an existing multi-unit building plus a DADU where a prefab tiny home option had already been selected.
That sounds ready, right?
Not fully. The first contractor questions were about the concrete base, foundation, retaining wall, new stairs, and whether the floor plan needed to expand. That is exactly why ADU and DADU planning needs contractor input before everyone assumes the drawings are done. A city-approved or almost-approved plan can still leave big construction questions sitting in the yard.
Prefab does not remove the lot. The lot still has grade. It still has water and sewer. It still has drainage. It still has access. It still has the awkward place where the stairs have to land. If the backyard cottage gets delivered but the foundation, retaining wall, side sewer and utility path are underdeveloped, the "simple prefab" project is not simple anymore.
This is why dadu design build seattle should mean design and construction talking early. If design runs ahead by itself and construction only shows up after permit, the homeowner is the one paying for every missing assumption.
What SDCI Says, in Homeowner Language
SDCI says ADUs "may not be subdivided from the main house and are not legal unless they have been established through a permit process." That is a big sentence. A basement with a bed and a kitchenette is not automatically a legal unit. A backyard office with a bathroom is not automatically a DADU. Legal status comes from the permit path.
SDCI also says accessory dwelling units are restricted in size and generally count toward density limits. They must comply with the property's zone height and location standards. SDCI says an ADU must meet current Seattle residential, building, mechanical, electrical, energy, land use, environmentally critical areas and shorelines code standards.
That is why adu permit drawings are not just a sketch with room names. The plans have to show the city what is being created, and they have to show the builder what is changing.
Useful official references include:
- SDCI Accessory Dwelling Units
- SDCI Tip 116A, Establishing an Attached Accessory Dwelling Unit
- SDCI Tip 116B, Establishing a Detached Accessory Dwelling Unit
- Updated Seattle ADU ordinance
- SDCI permit fee guidance
Those links are helpful, but they are not a substitute for looking at the actual property. Seattle has too many houses where the code issue is not theoretical. It is the beam in the basement, the narrow stair, the sewer line under the patio, the alley grade, the tree nobody wanted to talk about, or the window that almost works but does not meet egress.
Attached ADU Plans: Basement ADU, Lower-Level Studio, Garage Conversion
Attached ADU plans usually look less dramatic than a backyard cottage, but they can be more annoying in real life because the existing house is already full of decisions.
For a basement ADU, the biggest early questions are:
- Is there legal ceiling height where people will actually live?
- Is there emergency escape and rescue for sleeping rooms?
- Can the bathroom and kitchen plumbing route without insane trenching?
- Is the electrical panel ready for new loads?
- Is there a safe heating and ventilation strategy?
- Is moisture handled before drywall and flooring go in?
- Does the unit have a clear, legal, practical entrance?
- Can storage and mechanical access still function after the layout changes?
SDCI Tip 116A says AADUs need emergency escape and rescue openings in sleeping rooms that were built or converted by permit after Aug. 10, 1972. It says emergency escape windows must have an unobstructed opening of at least 5.7 square feet, with at-grade openings allowed to be 5 square feet. It also says the usable opening must be at least 24 inches tall and 20 inches wide, and the finished sill must not be more than 44 inches above the floor.
That one paragraph can become excavation, concrete cutting, a window well, drainage, framing, interior patching and inspection timing. That is why egress belongs in the budget early.
On one of our past lower-level conversion projects, the planning started with a rough layout because the owner wanted a legal bedroom, permits, a more livable lower level and a bathroom. The useful part was not pretending the first sketch was final. The useful part was involving planning input early enough that the estimate could be broken down line by line and then adjusted after the final plan set.
That is how we prefer to work. A ballpark number before drawings is a planning tool, not a promise. Once the drawings are finalized, Renova Contractors LLC reviews the plan set and updates the budget around what is actually shown.
DADU Plans: Backyard Cottage Design Is a Site Plan Problem
DADUs are emotionally easy to understand. A small home in the back. A guest house. A rental. A place for family. A quiet office that could later become housing.
Technically, they are much more serious. SDCI Tip 116B says a DADU is also known as a backyard cottage or mother-in-law unit. It also says DADUs are not permitted on lots in shoreline districts. Tip 116B states that a DADU must meet Seattle residential, building, stormwater, side sewer, mechanical, electrical and energy code requirements that apply to single-family dwellings. It also says a DADU is not an accessory structure for application of the Seattle residential and building codes.
That last part matters. A DADU is not a shed with better marketing. It is a dwelling unit.
For backyard cottage plans seattle, a serious set of drawings usually needs to answer:
- Where exactly does the structure sit on the lot?
- What are the side and rear setbacks?
- Does alley access help or create a grade problem?
- Where does the foundation go?
- How do water, sewer, power and drainage reach the unit?
- Does the project need retaining walls, stairs or special access?
- How does the DADU meet energy code?
- Where are the windows, egress points, heating controls and ventilation?
- How will construction materials physically reach the backyard?
Homeowners often want the floor plan first because that is the fun part. We get it. But the unsexy site plan is where the cost hides.
Standard Plans vs Custom Plans
Seattle has standard plan resources and ADUniverse options, and for the right property they can save time. But a standard plan is not a magic coupon. You still have to make it fit your specific lot.
A standard plan may work when the lot is simple, access is good, utilities are reasonable, the structure fits cleanly, and the homeowner is happy with the layout.
A custom plan is usually better when:
- the yard is tight or sloped
- the owner wants a specific layout
- there is alley access but difficult grade
- trees, decks, patios or retaining walls affect placement
- the sewer route is not obvious
- the cottage needs to match the house
- the plan has to preserve yard function
- the construction budget needs to be controlled around real site conditions
From our past plan-set coordination, the small changes before submittal are often the ones that keep the project from feeling wrong later. One homeowner asked to move a wall between a den and bedroom by 3-5 inches so the den would feel larger while the bedroom still worked. Another asked whether a laundry room could shift to allow a sink and stacked laundry. Another asked whether replacing a lower deck with about 300 square feet of pavers should be shown on the site plan. The designer's answer was basically: yes, show it on the site plan.
That is not overthinking. That is the permit package becoming honest.
The Architect Search: What Homeowners Should Actually Ask
If you are searching seattle adu architect, adu architect seattle, dadu architect seattle, or comparing seattle dadu architects, do not only ask for pretty examples. Ask buildability questions.
Ask:
- Have you designed ADUs or DADUs in Seattle specifically?
- How do you check setbacks, lot coverage and site constraints?
- When do you bring in a structural engineer?
- Will the plan include energy code information?
- How are sewer, water, drainage and electrical routes shown?
- How many revision rounds are included?
- Who responds to corrections?
- Does a contractor review the plans before submittal?
- Are the drawings permit-ready only, or also build-ready?
That last question matters. Permit-ready and build-ready are not always the same thing. A plan can pass review and still leave the contractor guessing about sequencing, access, utility conflicts, finish assumptions, or what exactly was included in the budget.
Renova Contractors LLC coordinates with licensed professionals when needed, but our job is to keep the design connected to construction. The architect or designer shapes the drawings. The engineer handles load path and structural requirements. The contractor prices and builds. The city reviews. When those roles are clear, the homeowner is less likely to get trapped between them.
Real Project Numbers That Help Set Expectations
ADU and DADU plans do not have one flat price because every property changes the work. But past project data gives useful anchors.
A smaller permit, drawing and engineering package tied to egress structural changes was around $5,700. A more serious lower-level conversion planning phase with architectural and permitting work was around $11,000 before construction. A full permitted lower-level conversion with bathroom, laundry, framing, plumbing, electrical, HVAC, insulation, drywall, floor assembly, flooring, bathroom completion, millwork, painting and bedroom egress had a $107,700 construction subtotal and $118,846.95 total with Seattle sales tax.
That was not a detached backyard cottage, but it is useful for ADU planning because it shows how fast a legal living-space conversion grows when drawings, permits, egress, utilities and inspections are real.
For bigger ADU-style scopes, one large older-home planning lead came through with an estimated range of $329,000-$486,000. The scope included egress, full bathroom, wet bar, electrical, drainage, permits, structural conditions and a planning timeline. We would never call that a final quote without walking the property. But it shows the right lesson: once a project becomes ADU-level living space, it is not priced like a cosmetic basement finish or a simple office.
The honest budgeting approach is:
- define ADU type first
- confirm feasibility before design gets expensive
- price permit drawings separately when needed
- identify structural and utility unknowns early
- update construction pricing after drawings are complete
- keep owner decisions in writing before submittal
That is how adu design build seattle should work. Design and build should not be two strangers meeting for the first time after permit approval.
Permit Fees and Timing: The City Costs Money Too
SDCI says permit cost depends on the size and complexity of the project, the value of the work, additional permits required, how long review takes, inspection fees and technology fees. SDCI also says that if a permit requires plan review, 75 percent of the total plan review and permit fees are paid when the permit application is accepted, and final fees are paid before permit issuance.
The 2026 SDCI fee summary gives a useful example: a 500 square foot DADU with project value $93,145 shows plan review and permit fee changing from $2,908 in 2025 to $3,453 in 2026.
That does not mean your DADU permit will be exactly $3,453. It means the city is very clearly telling homeowners that permits are a real cost, not a courtesy stamp.
SDCI also says it will report all ADU permits to King County for sewer treatment capacity charges and that during application you need to fill out a form acknowledging those capacity charges. This is one of those items people miss when they only budget for drawings and construction.
What a 10/10 ADU Plan Set Should Make Clear
A strong ADU or DADU plan set should help the city review the project and help the contractor build it. If it only does one of those, the homeowner still has a problem.
The set should make these items clear where applicable:
- existing conditions and measurements
- site plan with lot lines, structures, access and setbacks
- floor plans and room uses
- exterior elevations
- sections and ceiling heights
- foundation and framing information
- structural engineer notes where needed
- energy code information
- window and door schedules
- egress requirements
- smoke and carbon monoxide alarm locations
- mechanical ventilation and heating controls
- plumbing and side sewer coordination
- electrical and panel notes
- stormwater and drainage items
- tree, ECA or shoreline flags if applicable
- owner selections that affect rough-in
- construction scope tied to the estimate
For a basement ADU, the plans should be honest about beams, ducts, stairs, headroom, moisture, egress, mechanical access and utility rooms. For a DADU, the plans should be honest about the lot, foundation, sewer, drainage, setbacks, roof height, stairs, access and where the crew will actually work.
The best plans reduce surprises. They do not eliminate every surprise because construction is still construction, but they keep avoidable questions from becoming expensive field decisions.
The Human Part: Homeowners Are Not Being Difficult
When homeowners ask to move a wall a few inches, add a laundry sink, stack appliances, keep a patio, or adjust a bedroom door, they are not being annoying. They are trying to imagine living with the plan after everyone else leaves.
That is why we like to catch those questions before submittal. Once the permit package is assembled, every change has more gravity. Once construction starts, small changes start pulling on trades. A laundry sink can affect plumbing. A stacked washer can affect venting, electrical and cabinet layout. A bedroom wall shift can affect egress, closet clearance, switches and furniture. A paver patio can affect drainage and the site plan.
Good ADU planning respects the daily-life questions and the code questions at the same time.
How Renova Contractors LLC Coordinates the Process
Our process is straightforward:
- Walk the property and existing structure.
- Decide whether the project is AADU, basement ADU, attached ADU, DADU, garage conversion or backyard cottage.
- Review zoning, lot constraints, access, utilities, egress, ceiling height and structure.
- Coordinate with a licensed architect, designer and structural engineer when the scope requires it.
- Build the early scope in enough detail that the construction budget is not fantasy.
- Review drawings before submittal so the plan matches the likely build.
- Track owner decisions and revisions before they become expensive changes.
- Coordinate permit responses, corrections and final construction pricing.
The goal is not to make ADU planning sound easy. The goal is to make it organized enough that homeowners can make good decisions.
If you are early in the process, start with feasibility. If you already have drawings, start with buildability review. If you are comparing architects, ask how the plan connects to construction pricing. If you are comparing contractors, ask whether they understand the plan set or are just reacting to it.
ADUs and DADUs can be great projects in Seattle. They can create family flexibility, rental potential, guest space and long-term property value. But they punish vague planning. The sooner the architect, engineer, contractor and homeowner are working from the same reality, the better the project feels.
For help with plans, permit drawings and construction coordination, start with Renova Contractors LLC's Seattle architectural plans and permit coordination service.