Yes. Even though accessory dwelling units are allowed on most residential lots, every ADU still has to go through the Seattle permitting process. Zoning rules control height, setbacks, lot coverage, and how many units are allowed, and construction plans must be reviewed for structural, plumbing, electrical, and energy-code compliance before work can begin.

In Seattle, permit review typically takes two to five months, depending on zoning, site conditions, and whether corrections are required. Projects involving utility upgrades, drainage review, or environmental overlays can take longer, which is why ADU timelines are usually driven more by permitting than by construction

In many Seattle neighborhoods, yes. The city allows up to two ADUs on a single lot when size, height, and lot-coverage requirements are met. Whether both are allowed depends on the existing home, the lot layout, and how the units are designed.

Costs vary widely based on size, foundation type, utility connections, access, and finish level. Detached ADUs usually cost more than basement or attached units because they require new foundations, framing, and separate utility runs. In urban areas, site access and utility upgrades often have a bigger impact on cost than square footage alone.

Older homes may need electrical panel upgrades, larger water lines, or new sewer connections to safely support an additional dwelling. These requirements are typically identified during the design and permit phase and can significantly affect the overall budget.

Yes. Seattle allows ADUs to be used as long-term rental units. Short-term rentals, such as vacation rentals, have additional registration and compliance requirements and are regulated separately.

Yes. In Seattle, the county reassesses the added living space when an ADU is built, which increases the home’s taxable value. The increase is based on the value of the new unit, not simply the construction cost, and many homeowners offset it with rental income or higher resale value.

This can happen, especially on older or infill lots. If issues are found during excavation or inspection, the city may require engineering revisions or design changes. A properly planned ADU project accounts for these possibilities so they don’t become major delays or budget shocks.

Most ADUs in Seattle require architectural drawings, structural engineering, and energy-code documentation before permits can be issued. A full-service ADU builder coordinates these disciplines so the plans, permits, and construction stay aligned.

Detached ADUs have minimal impact on your living space, while basement or attached units require more access, noise, and utility work. On tighter lots, careful staging and scheduling are especially important to keep disruption manageable.

An ADU includes workmanship warranties on the construction itself, as well as manufacturer warranties on materials such as roofing, windows, siding, and appliances. Structural and installation quality are covered separately from product warranties.

Many homeowners use home-equity loans, cash-out refinancing, construction loans, or private financing. Because ADUs can generate rental income, some lenders treat them as income-producing improvements.

Most ADU builds take six to ten months after permits are issued, depending on size, site access, utility complexity, and inspection scheduling.

In most cases, yes. An ADU adds livable square footage and rental potential, which typically increases both market value and buyer demand. Homes with permitted ADUs often sell for more because they offer income, multigenerational living options, or flexible use of space.