Portland, Clackamas County, and Washington County all allow accessory dwelling units, and the demand for them keeps growing: a rental, a place for parents, or a home office that isn't the dining table. The first question everyone asks is what it costs, and the honest answer is that three very different projects hide under the same acronym.
A garage conversion is usually the least expensive path because the foundation, roof, and walls already exist. The money goes into insulation, plumbing and electrical service, a bathroom, a small kitchen, and finishes. A basement apartment sits in the middle: egress windows, moisture control, and a separate entrance add cost, but the structure is already there.
A detached ADU built from the ground up is the most expensive option and the most flexible one. Foundation, framing, roofing, siding, utilities trenched from the main house, and a full set of finishes are all new. Site conditions matter here more than anywhere else: a sloped lot, a long utility run, or a tree in the wrong place can move the total more than the cabinet choices will.
Whatever the type, three decisions drive most of the variance: the size of the bathroom and kitchen, the utility connections your property requires, and the finish level. A written, itemized estimate lets you see each of those separately and decide where the budget should go. Permit review time is real too, so build it into the schedule, not just the price.