ADUs & backyard cottages

Financing an ADU or backyard cottage when your home still has a mortgage.

Washington now requires most cities in urban growth areas to allow two accessory dwelling units on lots zoned for single-family homes, and Seattle has allowed backyard cottages (DADUs) for years. The harder question for most owners is money: how to borrow against a house that already has a first mortgage, and what the finished unit does to value and resale. This hub compares the financing routes and the steps in the right order.

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Detached backyard cottage behind a craftsman house at dusk — AI-generated concept illustration; not a photo of a specific place or property.

Quick answer

You can usually add an ADU to a house that has a mortgage — you do not have to pay the first loan off. The common routes are a HELOC or home equity loan (keeps your current rate, borrows against existing equity), a cash-out refinance (replaces the first mortgage, makes sense mainly when the new rate is acceptable), and a renovation loan such as Fannie Mae HomeStyle or FHA 203(k) that lends against the value after completion. Tell your insurer before construction, keep permits final, and expect the lender or appraiser to value the finished ADU using comparable sales, not build cost.

2 ADUs

Minimum most Washington cities in urban growth areas must allow per single-family lot (RCW 36.70A.681)

3 routes

HELOC / home equity loan, cash-out refinance, renovation loan

No owner-occupancy

State law bars cities from requiring the owner to live on the lot

Questions this hub answers

financing an adu
adu financing options
how to finance an adu
heloc for adu
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seattle adu rules
can i build an adu with a mortgage

Steps to fund and build an ADU

01
Check what your lot allows
Confirm ADU size limits, setbacks, parking and height with your city (Seattle SDCI, Bellevue Development Services, Redmond, Kirkland). Rules differ by city even under the state law.
02
Get a realistic budget
Ask a designer or builder for a range that includes design, permits, utility connections and impact fees. Utility and site work often surprise owners more than the structure itself.
03
Compare financing routes
Ask at least two lenders to quote a HELOC, a cash-out refinance and a renovation loan on the same budget. Compare total interest cost, draw rules and whether future rent can help you qualify.
04
Line up insurance
Tell your homeowners insurer before work starts and ask about builder's risk coverage. Your deed of trust requires you to keep the property insured.
05
Permit, build and final
Draw funds against inspections and keep receipts. A finalled permit matters later: unpermitted units are a common issue at resale.
06
Plan the use
Long-term rental, family housing or home office each affect taxes and insurance differently. Check landlord-tenant rules if you will rent it.

Which ADU financing route fits?

Route	Keeps your first mortgage?	Borrows against	Best when
HELOC	Yes	Current equity, variable rate	You have strong equity and a low existing rate
Home equity loan	Yes	Current equity, fixed payment	You want a fixed budget and payment
Cash-out refinance	No — replaces it	Current value	Today's rate is close to or below your existing rate
Renovation loan (HomeStyle, FHA 203(k))	Usually replaces it	Value after completion	Equity today is not enough for the project
Cash / savings	Yes	—	You want no new debt and can wait to save

Floor plan of a backyard cottage beside a calculator and house keys — AI-generated concept illustration; not a photo of a specific place or property.

Checklist

ADU financing checklist

City ADU rules confirmed for your address
Design and builder budget including utilities and fees
Quotes for HELOC, cash-out refinance and renovation loan
Draw schedule agreed with lender and builder
Insurer notified; builder's risk coverage in place
Permit issued before work starts
Final inspection passed and documents filed
Rental plan and landlord-tenant rules reviewed

ADU financing FAQ

Can I build an ADU if I still have a mortgage?

Usually yes. Most homeowners finance an ADU with a HELOC, home equity loan, cash-out refinance or renovation loan while keeping the property mortgaged. Read your deed of trust and tell your lender if a refinance is involved.

Does an ADU add value to my home?

A permitted, well-built ADU often adds value, but the appraiser uses comparable sales with similar units, not your build cost. In areas with few ADU sales, the added value can be harder to prove.

Can future rent help me qualify for the loan?

Some programs allow part of projected ADU rent to count as income under specific conditions. Ask each lender how they treat ADU rental income before you choose a route.

Do I have to live on the property?

Washington law bars cities in its scope from requiring the owner to live in the house or the ADU. Check your HOA covenants, which can still restrict rentals.

Sources

Last checked October 29, 2026. Official sources for the key rules on this page; they describe general rules, and your contract, lender, tax adviser and each agency's current tools control your specific case.

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Wondering what an ADU would do to your home's value?

Tell us your address and plan. We will look at local sales of homes with ADUs and share what buyers have paid for them, so you can size the project before borrowing.

Ask about ADU value