You can use cash, home equity, construction financing, or a mix. The best fit depends on reserves, payments, and how certain the project is.
THE SHORT ANSWER
Protect the project and the household.
Compare the money available now, payments during the build, what happens after completion, and the cash left for surprises. The lowest starting rate is only one part of the decision.
Paying from savings: no interest and no lender, but the money leaves your cushion for good. Compare what remains after the project plus an overrun reserve. Next: write down the amount you could spend and still handle a bad month.
HOME EQUITY
HELOC or equity loan
Two different products against your home’s equity. A HELOC is a line you draw on as bills come due, usually at a variable rate. An equity loan is one lump sum at a fixed rate from day one. Compare the rate type, what you pay during the build, and any charge for money you never draw. Next: ask your bank’s equity desk for both in writing.
PROJECT LOAN
Construction financing
A loan built for building: money is released in stages as work passes inspection, then converted or paid off at completion. Compare what plans and contractor the lender wants before closing, how draws are timed, and the payment once the build is done. Next: ask whether the lender finances an ADU on an existing home and what it needs to see first.
NEW MORTGAGE
Cash-out refinance
Replacing your whole mortgage with a larger one and taking the difference in cash. Compare the new rate against your current one on the entire balance, not only the new money, plus closing costs and the years added. Next: get the total monthly payment before and after, on paper.
ASSISTANCE PROGRAMS
Assistance programs
Grants or low-cost loans from public programs, each with eligibility and use rules. Compare who qualifies, what the money may pay for, any occupancy or rent rules that follow, and whether funding is open right now. Next: read the San Diego Housing Commission and CalHFA pages linked below for current terms.
RENOVATION MORTGAGE
Renovation loan
HomeStyle Renovation and CHOICERenovation combine your mortgage and ADU construction costs, using the home’s value after the work. HomeStyle can release up to half the renovation budget at closing, including design and permit fees. Not every lender offers them. Compare rates, fees, and payment timing. Next: ask lenders about Fannie Mae’s HomeStyle or Freddie Mac’s CHOICERenovation.
MIX AND MATCH
A blended plan
More than one source, each covering a stage: savings for design, a line for construction, a refinance at completion. Compare which source pays which stage and what reserve is left at the end. Next: map the four periods below against each source.
MODEL ALL FOUR
The loan has to work over time.
01Before construction
02During the build
03At completion
04Long term
COMMON QUESTIONS
What homeowners ask next.
Is a HELOC the best way to finance an ADU?
There is no universal best. Compare payment risk, available equity, project certainty, draw timing, reserves, and the household's existing mortgage before choosing.
Can rental income qualify me for the loan?
It can if you are buying a one-unit home you will live in, or refinancing it without taking cash out, and the home already has an ADU. Fannie Mae and Freddie Mac allow lenders to use part of that ADU’s rent to help you qualify. For rent documented with a lease, lenders can count 75% of the rent, capped at 30% of your total qualifying income. Fannie Mae counts rent from only one ADU. Freddie Mac does not count rent from an ADU that local zoning does not allow. These rules do not cover projected rent from an ADU you plan to build. Ask your lender how much rental income it can use for your application.
Are there San Diego ADU grants or low-cost programs?
Programs and funding status change. Use current official sources, confirm which city or county governs your property and whether you qualify, and do not rely on old pilot terms or contractor urgency claims.
Should I preserve cash and borrow more?
That is a household risk decision. Compare the value of reserves with loan cost, payment coverage, construction risk, and emergency needs.
