It’s feasible and increasingly common – Washington’s recent legislation made backyard cottages and garage conversions considerably easier to permit. Your detached garage is worth looking at first, because converting existing structure is usually faster and less expensive than building new. On timeline, most Bellevue projects run four to six months from permit submission to move-in, so a spring arrival means starting the process now rather than in February. New Day Construction handles design, engineering, permitting and build in one contract, details at http://www.newdayconstruction.co. Worth deciding early whether he needs full kitchen and accessible bathroom, since that shapes the whole design.