ADU and DADU Design
Coates Design plans and designs accessory dwelling units across Seattle, Bainbridge Island, Kitsap County, and the Eastside. Most projects start with a feasibility study that establishes what your lot actually allows before you commit to a design contract.
Start with a feasibility study
What we design
- Detached ADUs (DADUs). Backyard cottages and carriage houses as standalone structures, including two-unit configurations where zoning and lot coverage allow.
- Attached ADUs (AADUs). Units integrated into the main house, including additions designed to read as part of the original architecture rather than as an appendage.
- Basement and garage conversions. Adapting existing space, where ceiling height, egress, moisture, and energy code compliance determine whether the conversion is actually cheaper than building new.
- Constrained and complex sites. Sloped lots, critical areas, tree-protected yards, shoreline parcels, and septic-served properties where a standard footprint does not fit.
Where we work, and why jurisdiction matters
ADU rules are not regional. They are municipal, and they differ enough that the same design can be permittable in one city and not in the next one over.
| Jurisdiction | What tends to drive the project |
|---|---|
| City of Seattle | Floor area ratio and lot coverage limits, tree protection, and the choice between a pre-approved ADUniverse plan and custom design |
| Bainbridge Island | Septic capacity, steep slopes and forested lots, shoreline jurisdiction, and neighbor sightline considerations |
| Kitsap County | Septic and well capacity, county setback and size limits, and access on rural parcels |
| Eastside cities | Jurisdiction-specific size caps and design standards that vary meaningfully between neighboring cities |
If your property is outside these areas, tell us anyway. We will say plainly whether we are the right firm for it.
How the process works
1. Feasibility study
One to two weeks. We establish buildable area after setbacks, lot coverage, and tree protection; whether critical areas or shoreline jurisdiction apply; where utilities can reasonably run; and what the grade requires of a foundation. You come out of this knowing whether the unit you have in mind is possible, roughly what it will cost, and whether a pre-approved plan could work on your lot.
This is a small, defined scope. It exists so that nobody commits to a full design contract on a site that will not support the project.
2. Schematic design
Program, massing, and floor plan options. For an ADU this is largely a question of where the building sits and how it relates to the main house, the yard, and the neighbors. We typically present two or three approaches.
3. Design development
Materials, systems, structure coordination, and a reconciled budget. Energy code compliance is resolved here rather than discovered at permit review.
4. Construction documents
The permit and build set, coordinated with structural and civil consultants. This is the largest single block of work in any architectural engagement.
5. Permitting
Submittal, response to review comments, and coordination with the jurisdiction through issuance. Timelines vary substantially by city and by whether critical areas or tree review apply.
6. Bidding
Contractor selection, bid leveling, and scope clarification so that you are comparing comparable numbers.
7. Construction administration
Site visits, submittal review, RFIs, and change order review. This is the phase clients most often try to cut. It is also the phase where design intent gets protected and substitutions get caught before they are installed.
What it costs to work with us
Architectural fees for ADU projects in this market generally run as a percentage of construction cost, with the percentage higher than a full-size custom home because the coordination work does not scale down proportionally with square footage. Feasibility studies are billed separately as a fixed fee.
What sits outside the architectural fee, and what most first-time ADU budgets underestimate:
- Structural, civil, and geotechnical engineering
- Survey and, on many sites, a geotechnical investigation
- Utility connections, separate metering, and electrical service upgrades
- Sewer capacity charges and jurisdictional permit fees
- Site work, excavation, retaining, and drainage
On most ADU projects, site and utility work represents a larger share of total cost than the building itself. This is the single most common reason an ADU budget fails, and it is the reason we start with feasibility rather than with drawings.
For a fuller breakdown of how architectural fees are structured, see our guide to architect fees in Seattle.
When you should not hire us
If your lot is flat and unconstrained, your utilities are close, and you mainly need speed, a pre-approved plan from Seattle’s ADUniverse gallery is a rational choice and a custom design contract is not a good use of your money. We will tell you that in the feasibility study rather than after you have signed.
Custom design earns its fee when the site pushes back: slope, critical areas, tree protection, tight setbacks, difficult access, septic constraints, or a program that needs the ADU to work with the main house rather than sit behind it. Those are the projects we are built for.
More on that decision: Pre-Approved DADU Plans in Seattle: When to Use One.
Why Coates Design
Coates Design Architects + Interiors was founded in 2005 by Matthew Coates, AIA. The firm’s residential work centers on sustainable, regionally responsive design in the Pacific Northwest, and its permitting experience spans Seattle, Bainbridge Island, and the surrounding Puget Sound jurisdictions.
For ADU work specifically, three things matter more than portfolio aesthetics:
- Site judgment. ADUs succeed or fail on where the building goes. Most of the value an architect adds on a small structure happens before the first floor plan.
- Jurisdictional familiarity. Knowing what a specific reviewer will flag is the difference between one review cycle and three.
- Energy performance. Small buildings are harder to make efficient than large ones because the surface-to-volume ratio works against you. Washington’s energy code is demanding, and meeting it well takes design attention rather than a code checklist.
Frequently asked questions
How do I know if my lot can support an ADU?
A feasibility study answers this in one to two weeks. It establishes buildable area after setbacks, lot coverage, and tree protection, identifies whether critical areas or shoreline jurisdiction apply, and confirms utility feasibility. It is the cheapest way to find out before committing to design.
What is the difference between an ADU and a DADU?
A DADU is a detached accessory dwelling unit, a standalone structure on the same lot as the main house. An attached ADU shares a wall or is contained within the main house, including basement and garage conversions. Both are ADUs. Zoning treats them differently, and the cost profiles are very different.
Do you work on Bainbridge Island and in Kitsap County?
Yes. The firm is based on Bainbridge Island. Rules on the island and in unincorporated Kitsap County differ from Seattle’s, particularly around septic capacity, slope, and shoreline jurisdiction.
How long does an ADU project take from start to occupancy?
Design through permit issuance commonly runs several months, with construction adding roughly another year depending on scope and contractor availability. Site complexity and jurisdiction drive most of the variance. A feasibility study gives you a realistic schedule for your specific property rather than a generic range.
Can you convert my garage or basement into an ADU?
Often, yes. Whether it is worth doing is a separate question. Ceiling height, egress, moisture, foundation condition, and energy code compliance determine whether a conversion is genuinely cheaper than new construction. Sometimes it is. Sometimes the required upgrades cost more than building fresh, and we will tell you which case you are in.
Do you handle permitting, or do I?
We prepare and submit the permit application and respond to review comments through issuance. Permit fees and capacity charges are paid by the owner directly to the jurisdiction.
Can you design two ADUs on one lot?
State law and local implementing ordinances have opened this on many lots. Floor area ratio and lot coverage still govern, so two maximum-size units is rarely what a parcel actually supports. Feasibility work establishes the real answer for your property.
Start here
Tell us your address and what you are trying to accomplish. If a feasibility study is the right first step, we will scope it. If your project is better served by a pre-approved plan or a different firm, we will say so.