ADU Financing: How Homeowners Actually Pay for One in 2026

Most ADU financing articles give you a list. “Option 1: HELOC. Option 2: cash-out refinance. Option 3: personal loan.” Then they describe each option in two sentences and send you to a lender. That’s not useful if you’re trying to figure out which one fits your actual situation.

This article covers the five products most homeowners use in 2026, with current rates, specific lender names, and the conditions that make each product the right or wrong call. Rates are sourced as of May 2026 or clearly date-stamped when live data wasn’t available.

If you haven’t nailed down what the build will cost yet, read the national ADU cost guide first, then come back. The financing math depends on knowing whether you’re looking at $180,000 or $320,000.

The five products

Most homeowners fund an ADU through one of five approaches. Local grant and subsidized-loan programs are a sixth path and get their own section below.

1. HELOC (home equity line of credit)

A HELOC is the most common ADU financing product, and for straightforward reasons. You borrow against existing equity, draw funds as construction progresses, and pay interest only on what’s outstanding. On a project that takes 8-12 months to build, you’re not paying interest on the full loan from day one.

The national average HELOC rate is 7.10% APR as of April 29, 2026, per Bankrate’s survey of major lenders (benchmarked at 80% combined LTV, 700 FICO, $30k line). Borrowers with strong credit and lower LTV can do better. Borrowers above 80% LTV or with credit below 720 pay more.

The standard ceiling is 80% combined loan-to-value (CLTV), meaning your mortgage plus the HELOC can’t exceed 80% of what your home is worth. On a $750,000 home with a $350,000 mortgage, you can draw up to $250,000 ($750k x 80% = $600k, minus $350k owed). That math works for many homeowners, but not for recent buyers with a small equity cushion.

For California homeowners specifically, Patelco Credit Union’s ADU HELOC raises that ceiling. As of May 1, 2026, the rate is 8.25% APR and the product allows up to 125% CLTV (or 90% of after-renovation value, minus what’s owed). Patelco also factors projected ADU rental income into qualification. The structure is a 2-year draw period followed by a 20-year repayment period, with interest-only minimums during the draw. California primary residences only. Source: patelco.org/credit-cards-and-loans/home-equity/adu-line-of-credit, rate as of May 1, 2026.

What to watch for: HELOC rates are variable. If you open a line at 7.5% and rates climb 1% during a 10-month build, your effective carry cost is higher than you modeled. Budget for that possibility when you estimate interest carry.

2. Cash-out refinance

A cash-out refinance replaces your existing mortgage with a larger one and gives you the difference at closing. Refinance a $300,000 mortgage on a $750,000 home to $520,000 and you walk away with $220,000 in cash.

The 30-year fixed rate averaged 6.30% for the week of April 30, 2026, per the Freddie Mac Primary Mortgage Market Survey. Cash-out refinances carry a small premium over rate-and-term refinances, so most borrowers should expect something in the 6.5-7.0% APR range today.

This product only makes sense if your current mortgage rate is at or above where the market sits now. If you bought in 2020 or 2021 at 2.75-3.25%, a cash-out refi locks your entire balance into a rate more than double what you currently carry. The additional interest cost on the original balance will outpace what the ADU earns in rent for years. That math doesn’t work.

If you bought or refinanced in 2022-2024 at 6.5-7.5%, the gap is much smaller. At that point it’s worth running the break-even: how long until ADU rental income offsets the additional interest cost on the full balance? If the answer is under five years and you plan to stay in the house long-term, model it seriously.

Most conventional lenders cap cash-out refis at 80% LTV. Some portfolio lenders and credit unions go higher.

3. Construction-to-permanent loan

A construction-to-permanent (“C-to-P”) loan is purpose-built for new construction. During the build phase, the lender disburses funds in draws tied to inspection milestones: foundation complete, framing complete, rough mechanical complete, and so on. You pay interest only on the drawn amount. At project completion, the loan converts to a standard permanent mortgage.

Construction-period rates for C-to-P loans run roughly 8-10% for conventional market-rate products (as of late 2025; confirm with lenders before closing). The permanent rate is set at prevailing market rates at conversion, not at the time you closed the construction loan.

The draw schedule is both the product’s main feature and its biggest operating constraint. Your general contractor needs to carry float between draws. If they can’t bridge the gap between inspection sign-off and the lender’s disbursement, the project schedule stalls. Nail down draw timelines and what your GC needs before you close the loan.

Fewer lenders offer C-to-P specifically for ADU projects. Community banks and credit unions with construction loan desks are the better targets. Most online mortgage platforms stick to purchase and refinance products and don’t handle construction draws well.

For a broader view of how a new-build ADU moves from financing through construction to certificate of occupancy, the ADU build walkthrough covers the full eight-phase timeline and the inspection milestones that drive draw schedules.

4. ADU-specific credit union products

A handful of credit unions have built ADU-specific financing structures that sit outside the standard HELOC or C-to-P categories.

Patelco’s ADU HELOC (described above) is the most-cited example in California. The 125% CLTV ceiling is the defining feature. On a home worth $500,000 with a $250,000 mortgage, the standard 80% CLTV leaves you $150,000. At 125% CLTV, Patelco’s formula allows up to $375,000 ($500k x 125% = $625k, minus $250k owed). That covers a full ADU project in Sacramento or the Inland Empire without requiring a paid-down mortgage or a high-value home.

For San Diego homeowners, the San Diego Housing Commission (SDHC) runs a separate construction-to-permanent loan: up to $250,000 at 1% interest during construction, converting to a 4% fixed rate for 15 years after completion. Source: adu.sdhc.org. The catch is a 7-year affordability covenant requiring you to rent the unit at below-market rate. If you’re planning a full market-rate rental, price in what the rate differential costs over seven years before applying. If you’d be close to affordable-rate rents regardless, this is one of the best-priced ADU products available anywhere in the country.

Outside California, Massachusetts has a new option. MassHousing launched an ADU construction loan program in January 2026 (masshousing.com) with a 20-year amortization term, structured as a combination of an interest-bearing portion and a zero-interest deferred portion. Rates weren’t publicly posted at launch; contact MassHousing directly for current terms.

5. Cash plus small-loan top-up

For homeowners with $150,000 or more in liquid savings, the simplest approach is often the cheapest: pay most of the project in cash, then carry a modest HELOC or personal loan to cover cost overruns and last-mile finishes.

[IMAGE NEEDED: Side-by-side comparison chart of the five financing products, showing rate range, maximum typical amount, equity required, best-fit situation, and one key watch-out for each]

On a $275,000 project with $210,000 in cash, the HELOC draw might be $65,000-$80,000. At 8%, interest-only on an $80,000 line is $533/month. Over an 18-month build with a draw that ramps gradually, total interest carry runs roughly $7,000-$9,000. Compare that to fully financing the same project at 8%: 18 months of interest on an average balance of $140,000 runs closer to $17,000-$20,000.

The risk is liquidity. Construction projects run over. If the rainy-day fund and the ADU fund are the same money, a $30,000 cost overrun creates real financial pressure mid-build. Keep at least 3-6 months of living expenses separate from any project account before construction starts.

Grants and subsidized programs

These programs are geographically specific and often underfunded. Worth knowing; not worth depending on until you’ve confirmed current availability.

CalHFA ADU Grant Program (California)

CalHFA (California Housing Finance Agency) offers up to $40,000 to reimburse pre-development costs: architectural drawings, permits, soils reports, surveys, and energy compliance reports. These are the costs incurred before construction starts and before most loans can disburse. Income limit is at or below 80% of area median income (80% AMI), which varies by county: roughly $84,160 for a household in Los Angeles or Orange County, $143,040 in Santa Clara County. Source: calhfa.ca.gov/adu.

Current status: paused. The last funding round was fully allocated as of December 2023. A new round had not been confirmed as of the time this article was written. Check calhfa.ca.gov before counting this money in your budget. If a round is open, apply immediately; previous rounds closed in weeks once word spread. If no round is open, build a budget that works without it and treat any future grant as upside.

San Diego Housing Commission (SDHC)

Described in Section 4 above. The 1%/4% rate structure is the best deal in the market for homeowners who qualify and accept the 7-year affordability covenant. Source: adu.sdhc.org.

MassHousing (Massachusetts)

Launched January 2026. Contact MassHousing directly at masshousing.com. The program details are still being implemented at the lender level.

What the Fannie Mae rule change means for qualifying

In October 2025, Fannie Mae issued Selling Guide update SEL-2025-08, effective October 8, 2025. For the first time, lenders following standard Fannie guidelines can count projected ADU rental income toward mortgage qualification on purchase transactions and limited cash-out refinances for a one-unit primary residence.

The cap: projected ADU income cannot exceed 30% of the borrower’s total qualifying income. If your documented income is $10,000/month, the ADU projection can add up to $3,000/month to your qualification picture. Documentation requires an appraisal showing fair market rent (Form 1007 or Form 1025), not your own estimate. Automated underwriting support through Fannie’s Desktop Underwriter v12.1 went live in Q1 2026.

What this changes in practice: a homeowner who’s a tight fit for a cash-out refi because the larger loan balance strains the debt-to-income ratio may now qualify if the ADU rental projection is credible and properly documented. It doesn’t lower your rate. It doesn’t apply to the construction loan itself. It applies to the qualifying calculation for the permanent financing.

For California homeowners, Patelco’s ADU HELOC was already factoring projected rental income into its qualification formula before the Fannie update. The October 2025 change brings that approach to conventional lenders across every market.

For context on what that projected rental income might realistically be in California metros, the California ADU rental income guide covers market-rate ranges from the Bay Area to the Central Valley, with cash-flow math on a typical $280-$320k build.

What to budget for interest carry

Financing a $300,000 ADU is not free money regardless of which product you use. Here’s the carry cost arithmetic for the three most common paths:

Full HELOC at 8%, 18-month build: average balance of $150,000 (ramping from $0 to $300k) at 8% for 1.5 years comes to roughly $18,000 in construction-period interest. That does not appear in any contractor’s quote.

Cash-out refi at 6.5%: no separate construction interest, but you’re now paying 6.5% on a balance $300,000 larger. Over 30 years, the additional interest on that $300,000 draw is roughly $383,000 in total interest paid, offset year over year by the rental income.

Cash plus small HELOC ($80k at 8%): roughly $8,000-$10,000 in interest during an 18-month build on the drawn portion. Cheapest carry of the three scenarios.

None of these are reasons to avoid the project. They’re line items to build into the budget before construction starts. The California ADU cost guide shows how construction loan interest appears in the regional cost tables.

Decision tree: which product to start with

Your situation	Start with
Current mortgage rate below 5%	HELOC or ADU-specific credit union product. Don’t refinance your full balance.
Current mortgage rate 6.5%+ and substantial equity	Model cash-out refi. Run break-even vs. HELOC before deciding.
Detached new build, prefer one loan closing	Construction-to-permanent. Find a lender who has closed ADU projects before.
California home, equity under 80% CLTV	Patelco ADU HELOC. 125% CLTV is the differentiator.
San Diego, comfortable with affordability covenant	SDHC program. 1%/4% rate structure won’t be matched by a conventional lender.
$150k+ in savings, project under $260k	Cash plus small HELOC top-up. Lowest total interest carry.
California, household income at or below 80% AMI	Check CalHFA grant status at calhfa.ca.gov first. Layer financing on top if a round is open.
Massachusetts	Contact MassHousing. New construction loan program launched January 2026.

One thing the table doesn’t capture: the lender matters as much as the product type. Find a mortgage broker or loan officer who has closed ADU-specific financing in the last 12 months. The appraisal process for an ADU project, especially after-renovation value estimates and rental income projections for SEL-2025-08 documentation, is different from a standard purchase loan. An inexperienced underwriter can stall a clean deal.

The bottom line

Most homeowners who finish an ADU use one of three paths: a HELOC against existing equity, a cash-out refinance when the rate math works, or cash plus a modest credit line for overruns. The ADU-specific credit union products matter most for California homeowners with limited equity or projects where projected rental income is needed to qualify.

Rates change and programs open and close. The figures in this article reflect May 2026. If you’re reading this later, re-check HELOC rates at bankrate.com, Patelco’s current ADU HELOC rate at patelco.org, and CalHFA grant status at calhfa.ca.gov before you build your budget.