A backyard cottage estimate is a list of scope decisions, and two similar totals can describe very different projects. This walks through design, permits and county charges, site staging, building systems, utilities, allowances, general conditions, sales tax, contingency and exclusions, with questions to ask every bidder.
The same backyard cottage plan can carry very different budgets on two Seattle lots. This walks through slope and access, utility runs and King County charges, standards that do not apply to ADUs, design shape, pre-approved plans, labor and finish allowances, with a checklist before asking for estimates.
Mercer Island runs on interim ADU rules with a shared floor area budget, parking tied to lot size and a wet season limit on steep lots. Check the land first.
Renton allows two ADUs per lot and offers free pre-approved model plans. Its size cap, parking rule and site permits still differ from Seattle’s.
The City of Shoreline sets its own ADU code, separate from Seattle and from shoreline environmental rules. Two units, sprinklers and no stock plans stand out.
Redmond allows two ADUs or tiny homes on each residential lot. Its size cap, tiny home rules and sprinkler requirement differ from what Seattle owners know.
Kirkland allows two ADUs on most lots and offers pre-approved DADU plans, but shoreline lots, parking and a recorded covenant set it apart from Seattle.
Bellevue allows two ADUs on most residential lots, but its size cap, parking rule and permit path differ from Seattle. Start with the Land Use Code.
Washington law sets a shared floor for ADUs, but each city near Seattle writes its own code. Size caps, parking and permit portals change at the city line.
When every builder seems capable, look at how each one handles your lot, talks about risk and answers in writing. That is where the real differences show.