Building an ADU costs $150,000–$400,000 in California. Most homeowners don't have that sitting in cash. This guide covers every realistic financing path — what each costs, what you qualify for, and where specific lenders are competitive in 2026.
Despite a small army of fintech "ADU loan" startups, almost every Californian who builds an ADU uses one of five financing structures: HELOC, cash-out refinance, dedicated construction loan, ADU-specific renovation loan that includes projected rental income in qualification, or a state grant program (CalHFA's ADU Grant Program when funded). Cash purchases happen but are rare under $250k builds.
Each has a different risk profile, draw schedule, and underwriting standard. The right one depends on three things: how much equity you have in the main house, your income relative to the new debt, and how much of the project cost the lender will recognise as added property value.
If you've owned your home long enough to have $200k+ in equity beyond your existing mortgage, a HELOC is the simplest path. You draw against the equity as the project progresses and only pay interest on what you've drawn. Most CA banks (BofA, Chase, Wells Fargo, Citi) and credit unions (Patelco, Provident, SchoolsFirst) offer HELOCs with combined-loan-to-value up to 80% — meaning if your house is worth $1M and you owe $500k, you can pull $300k.
Variable interest rates as of April 2026 are running 8.5–10.5% — pricier than they were 18 months ago but the cheapest option for most homeowners. Closing costs are typically $500–$2,000 (much lower than refinancing). The downside: variable rate, and if rates spike during your build you could see payments jump 30–50%.
Best for: homeowners with substantial equity, comfort with variable-rate exposure, and timeline flexibility (you can pause draws if rates spike).
Refinance your existing mortgage to a higher balance, take the difference in cash. Works well if your current mortgage rate is below market — you'd be refinancing into a higher rate, but locking in a fixed rate on the whole package.
If your current mortgage is at 6%+ (post-2022 origination), a cash-out refi at 6.8–7.4% (April 2026 30-year fixed for jumbo loans typical in CA) might actually save money on the existing mortgage portion AND fund the ADU. If your existing mortgage is sub-4% (pre-2022), this path is rarely a fit — you'd surrender your low rate.
Closing costs run 2–5% of the new total loan. On a $1M refi, that's $20k–$50k upfront — meaningful enough to skip this path if the spread vs HELOC is small.
A purpose-built loan for the ADU project itself. Funds disburse in stages tied to construction milestones (foundation poured, framing complete, dried-in, etc.). Interest accrues only on disbursed amounts during the build period.
At completion, the construction loan converts to a permanent mortgage (often called a "single-close construction-to-perm") OR you refinance into a new long-term loan. Construction-to-perm avoids two sets of closing costs.
California lenders active in this space: First Republic (now JPMorgan), Bank of the West (now BMO), Patelco, Provident, Self-Help Federal Credit Union, and a handful of specialty lenders like RenoFi and Hometap (the latter two are home-equity-investment products, technically different but compete for the same homeowners).
Construction loans require detailed contractor bids, fixed-price contracts, and inspector sign-offs at each draw. They're more administratively intensive than HELOCs but can fund 100% of construction costs (whereas HELOCs cap at 80% CLTV).
The newest category. Lenders underwrite the loan considering not just your current income but the projected rental income from the completed ADU. This dramatically expands borrowing capacity for homeowners who are tight on income relative to the build cost.
Fannie Mae's HomeStyle Renovation loan and Freddie Mac's CHOICERenovation both allow ADU income consideration. A handful of California lenders specialise: Theodore Capital, Renofi, Avant, and Patelco's ADU Loan product. Underwriting is stricter — they require an appraisal that values the property both as-is and as-completed-with-ADU, plus a rent comp study.
Rates are typically 50–100 basis points above standard mortgage rates. Loan-to-value can reach 95% of as-completed value (vs 80% on a HELOC). This is the right path if you're income-constrained but the ADU's projected rent will materially improve qualification.
California Housing Finance Agency runs an ADU Grant Program that offers up to $40,000 to eligible homeowners for pre-development costs (architectural plans, permits, soils tests, surveys). It's grant money — not a loan — so it doesn't repay.
The catch: the program has been periodically funded and unfunded depending on state budget cycles. As of April 2026 it is paused pending the FY2026-27 budget cycle. Check calhfa.ca.gov/adu before counting on it. When funded, it's first-come-first-served and tends to exhaust within weeks of opening.
Income limits apply (typically tied to area median income — varies by county). Even when funded, $40k won't cover construction — it's specifically for pre-development costs. Pair it with one of the four loan paths above.
For HELOC and cash-out refi: equity in the main house, current debt-to-income ratio (target under 43%), credit score (720+ for best rates, anything below 680 limits options).
For construction loans: detailed builder bid (fixed-price preferred), contractor's license verification, project timeline, draw schedule, your ability to cover any cost overruns out of pocket.
For ADU-income loans: as-completed appraisal with rent-comp study, your income (still required, just supplemented by projected rent), and the local rental market data showing that rent is achievable.
All paths: a real construction contract with start/completion dates and milestone schedule. "I'm thinking about an ADU" is not a fundable application.
Quick comparison at April 2026 rates for a $300,000 borrow against a $1.2M California home with $400k existing mortgage:
HELOC: 9.0% variable, $0 down, monthly interest-only during build ~$2,250/mo, principal+interest after build period varies.
Cash-out refi to $700k total: 7.0% fixed 30-year, monthly payment on the entire $700k ~$4,660/mo (vs ~$2,400 on the existing $400k @ 6%), so the marginal cost of the $300k pull is ~$2,260/mo.
Construction-to-perm: 7.5% fixed 30-year on $300k after conversion = ~$2,100/mo. Plus $5–8k extra closing costs vs a HELOC.
ADU-income loan @ 7.8% on $300k: ~$2,160/mo. Net of $2,500/mo expected rent, the ADU pays for itself with ~$340/mo of housing-cost reduction.
Numbers will move with rates — this is the shape, not the snapshot.
Underestimating the contingency. 10–15% of contract value should sit in reserve. ADU projects routinely surface $20k–$60k of unforeseen costs (foundation issues, utility hookup surprises, permit conditions). Lenders won't fund overruns — that's your problem.
Drawing the HELOC before the build starts. Interest accrues from the moment you draw. Time the first draw to actual material orders, not to permit approval.
Locking in a fixed-price contract too cheap. A bid that's $40k below the next-cheapest is usually a contractor planning to make up the gap with change orders. Lenders won't fund change orders.
Expecting the bank to value the ADU at construction cost. Appraisals look at comparable rented or sold ADUs in your area. If yours is the first detached ADU on your street, the appraised value may be 15–30% below what you spent. This kills LTV-based products.
Get a real cost estimate for your specific project — see our cost calculator.
Pull your home's current valuation (Zillow + Redfin + a real comp from a local realtor) — gives you the equity number HELOC and refi conversations start with.
Talk to two lenders: one credit union (typically the most flexible on construction loans) and one specialty ADU lender (best for ADU-income qualification).
Don't sign a construction contract until you have written loan approval — the loan amount may not match your dream design.
FHA's standard loan program isn't well-suited to ADU construction — it focuses on owner-occupied 1-4 unit purchases. FHA does have a 203(k) renovation program that can include ADU work, but it caps loan amounts and requires HUD-approved consultants. Most CA homeowners doing ADUs go conventional.
The refinance itself doesn't trigger reassessment under Prop 13. The ADU construction does — California reassesses the added value of the new structure. Expect roughly $2,500–$3,500/year in additional property tax for a $250k ADU.
401(k) loans (up to $50k) and IRA early withdrawals are technically possible but rarely the right move for ADU financing. The opportunity cost of liquidating long-term tax-advantaged growth almost always exceeds the interest cost on a HELOC or construction loan.
Most lenders treat this favourably — you live in the ADU, rent the main house. Owner-occupancy requirements are met, rental income from the main house counts toward qualification. Some lenders prefer this scenario because rental on a 4BR house is more proven than rental on a new ADU.
Often, yes — particularly Patelco, Provident, SchoolsFirst, and Star One for HELOCs and construction loans. Credit unions typically offer 25–75 basis points lower rates than the big banks for borrowers with strong credit. Worth getting two quotes.