Move-in and ongoing operating costs begin

What turns on at move-in

A few recurring costs and obligations begin once the cottage is occupied:

King County Wastewater Capacity Charge

King County bills the new sewer connection separately from anything SPU sends. The 2026 rate is $77.99 per Residential Customer Equivalent (RCE) per month. King County applies a reduced rate of 0.59 RCE for an ADU/DADU, recognizing the smaller wastewater load.

At 2026 rates, the bill works out to about $46 per month, billed quarterly, for 15 years — roughly $8,300 nominal in total. A lump-sum prepayment is available at a discount.

SDCI auto-reports the new ADU permit to King County, so the bill arrives without any action from you. Build it into the rental pro forma if the cottage is for rent.

RRIO registration (if rented)

Seattle's Rental Registration and Inspection Ordinance (RRIO) requires every rental unit in the city to register and pass periodic inspections. If you rent the DADU, you'll register through the Seattle Services Portal.

STR licensing (if Airbnb / short-term rental)

Short-term rentals are stricter than most owners assume. Most operators are capped at two STR units (primary + one secondary), with a 6-month annual occupancy requirement on the primary. RRIO and STR licensing are separate permits on top of the construction permit.

Tenant Relocation Assistance (TRAO)

Worth understanding before the first lease is signed. Seattle's TRAO requires landlords ending a tenancy under certain conditions to pay relocation assistance — a few thousand dollars for low-income tenants and up to two months' rent for others. If you build the DADU planning eventual family use but rent it in the interim, plan the lease term and end-of-tenancy approach with TRAO in mind from the start.

Insurance switch

Builder's Risk policy ends; homeowner / landlord / dwelling fire policy begins. Notify your carrier.

You're done

The cottage is built, legally occupied, and producing whatever value it was supposed to — rental income, family living space, home-office quiet, the equity gain.

The 12-to-24-month project is over.

The 15-year King County bill has just started.