A DADU (detached accessory dwelling unit) can indeed create cash flow for owners, but roughly 90% of Eastside homes in the greater Seattle area can't actually build one due to lot size, slope, zoning restrictions, and municipal requirements like fire access, parking, and setbacks. If you want to profit from a DADU when buying, you must do a feasibility study before making an offer — otherwise you may discover after purchase that your backyard can't be built on at all.
A DADU Isn't Build-Whenever-You-Want
In recent years, DADUs have been hyped by many as a cash-flow machine: build a small detached cottage in the backyard and rent it for extra income. But the harsh reality is roughly 90% of Eastside homes don't meet the build conditions. Treating 'the backyard can generate rent' as a hidden buying premise often falls through — buyers may even pay an extra premium for an impossible expectation.
Hard Prerequisites for Building
Building a DADU requires meeting a cascade of hard requirements: a large enough lot, not-too-steep slope, setbacks, compliant fire access, parking, and conformance with local zoning. Any single failure derails the project. Many Eastside homes have smaller lots and prevalent slopes, making simultaneous fulfillment of all conditions difficult — that's exactly why roughly 90% can't proceed. In other words, what determines whether an addition can be built isn't whether policy allows it, but whether the specific parcel passes these technical requirements.
The Gap Between Policy Intent and Reality
This ADU wave started with policy liberalization. In 2020, the City of Seattle passed local law allowing ADU construction and separate sale on all single-family lots — theoretically greatly expanding housing supply. In 2023, Washington State passed HB1337, requiring all cities in the greater Seattle metro area to have their own rules allowing ADU construction and separate sale by June 2025. Policy direction is clear, aiming to ease long-term supply shortages — but between theoretical supply potential and actually buildable projects there's a huge gap. Only a minority of lots truly meets all conditions.
Rules Vary by City
More complexity: zoning and municipal rules vary by city. Bellevue, Kirkland, and Redmond each have their own requirements — a plan workable in one city may not work in another. The hardest to get around is the lot-split restriction clause in title reports — these restrictions are permanent and irremovable. Subdivisions developed after 1960 typically have developer-inserted deed prohibitions on splitting; thus Kirkland, which developed primarily in the 1970s, has only ~1,000 new ADUs in the past four years — far fewer than Seattle, which was largely developed before 1950 and has minimal split restrictions.
What a Feasibility Study Should Cover
A proper feasibility study should check at minimum three levels. First, title level: confirm the title report has no lot-split restriction clauses. Second, lot level: verify lot size, slope, setbacks, and buildable area meet requirements. Third, municipal level: confirm fire access, parking configuration, and zoning all comply. Only when all three levels pass simultaneously does a DADU income expectation have a foundation. The cost of this study is negligible compared to the potential tens of thousands to hundreds of thousands of dollars in losses from buying the wrong home — a step no buyer targeting additions should skip.
Don't Be Fooled by the Surface
Many buyers act impulsively upon hearing about the ADU dividend, fantasizing about backyard rental income while ignoring the foundational prerequisites. Properties truly suited for DADU potential are in Shoreline, Everett, Renton, Lynnwood, Tacoma, Lake Forest Park and similar cities that started large-scale development before 1960 and have fewer deed restrictions — not the Eastside where deed restrictions are common. Buyers should put feasibility ahead of price: first confirm whether it can be built, then discuss whether it's worth buying.
