ADUs — accessory dwelling units — have become one of the more discussed residential projects in Washington, and the regulatory picture has moved considerably. That movement is itself the most important thing to understand.
Confirm current rules for your address
Washington has been broadly moving toward enabling ADUs, but implementation happens at city and county level and the details differ.
Because this area has changed, guidance written even a couple of years ago may describe requirements that no longer apply — in either direction. Confirm the current position with your jurisdiction before spending anything on design.
Remember which jurisdiction that is. In Seattle it is SDCI; in Bellevue, Tacoma, Everett or Spokane it is that city; only unincorporated areas go through the county.
The three forms
Interior conversion. Converting existing space — a basement, an attic, part of the house — into a separate unit. Usually the least expensive because the shell exists, though egress and ceiling height requirements frequently drive the work.
Attached. An addition to the existing house containing a separate unit. Shares a wall and often utilities.
Detached. A separate structure. Most expensive because it needs its own foundation, envelope and utility connections, and most flexible in layout.
What actually determines feasibility
Utilities. Frequently the deciding factor. Water, sewer and electrical service to the property must support an additional dwelling. On septic, capacity is a genuine constraint and involves the health department. Electrical service may need upgrading, which in older Washington homes is common.
Site constraints. Setbacks, lot coverage, height limits, and access. Critical area designations — steep slopes, wetlands, shoreline — can materially restrict what is possible, and Washington has plenty of all three.
Parking. Requirements have been changing and vary by jurisdiction and location.
The basement conversion trap
Worth its own mention because it is the most commonly underestimated route.
A basement conversion sounds cheap because the space exists. What frequently makes it expensive is egress — a bedroom needs a window meeting minimum clear opening dimensions and maximum sill height, and basement windows generally do not.
Achieving compliance usually means enlarging the opening and installing an egress well outside it, which involves excavation, structural alteration and permits. Ceiling height is the other common obstacle, and there is very little that can be done about it economically.
Establish both before budgeting, not after.
Costs to plan for beyond construction
Design and engineering. Permit and impact fees, which vary considerably by jurisdiction. Utility connection or upgrade work. Site work for detached units — foundations, trenching, access.
These are real and frequently omitted from early budgets, which is how ADU projects develop a reputation for exceeding expectations.
Choosing who builds it
An ADU is a complete dwelling — foundation, envelope, plumbing, electrical, ventilation, finishes. It needs a contractor who builds houses rather than one who does rooms.
Verify registration, bond and insurance as with any project, and ask specifically for recent ADU work in your jurisdiction. Local permitting experience matters here more than most projects, because the process is where these get delayed.
Start with the feasibility question
Before design, before budget, before anything: can an ADU legally be built on your lot, at what size, and can the utilities support it? Getting that answered first prevents the most expensive kind of wasted effort.