ADU Financing in California: How to Pay for the Build

Accessory dwelling units have become one of the most common home improvement projects in the East Bay. State law has opened the door for most single-family lots to add a backyard cottage, a garage conversion, or a junior ADU carved out of the existing house, and homeowners are using that space for aging parents, adult children, and rental income. The part that usually takes the most planning is the money.

ADU financing in California comes in several forms, and the best fit depends on how much equity is already in the home, what kind of unit is going in, and where the existing first mortgage stands. This guide focuses on paying for the build. For the separate question of using ADU rent to help qualify for a mortgage, our post on ADU rental income for FHA loans covers that in detail.

California has spent the last several years loosening ADU rules at the state level, which is a big part of why construction has picked up across Contra Costa and Alameda counties. On most single-family lots, a homeowner can generally add one detached or attached ADU plus one junior ADU. A junior ADU is a smaller unit, capped at 500 square feet and created within the walls of the existing home, often from a bedroom suite or similar space.

Setbacks, utility connections, and permitting timelines still vary by city, so plans and permits usually come together before any financing decision is final. Worth knowing: the type of unit matters for financing too. Some loan programs treat a garage conversion or attached addition differently from a brand-new detached structure, which comes up again below.

California updates its ADU laws often, and cities adjust their local rules to match, so the city planning department is the best source for what applies to a specific lot today.

There are four common ways to fund an ADU project, and many homeowners end up combining two of them. Each type of ADU loan is sized from a different number, which is where most of the planning happens.

Cash-Out Refinance

A cash-out refinance replaces the existing first mortgage with a larger one and pays out the difference at closing. The full budget is in hand before construction starts, which keeps paying the contractor simple.

This option is sized on the home’s current value rather than its value with the ADU, so it fits best when there is already substantial equity. It also sets a new rate on the full balance. For owners who locked in a low rate a few years ago, that is worth weighing against the options that leave the first mortgage alone.

HELOC or Home Equity Loan

A home equity line of credit sits behind the existing first mortgage, so the original loan and its rate stay in place. With a HELOC, funds are drawn as each phase of the build is billed, and interest is charged only on what has been drawn. A home equity loan works similarly but pays out a lump sum, usually at a fixed rate.

Like a cash-out refinance, a HELOC is typically based on current value. It tends to suit smaller projects, such as a junior ADU or a garage conversion, and owners with plenty of existing equity. Most HELOCs carry a variable rate, which is worth factoring into the budget for a balance that may take several years to pay down.

Renovation Loan Based on After-Completion Value

A renovation loan is often the answer when a home has strong value but not quite enough equity to cover the full build. These loans are underwritten using the after-completion value, meaning the appraiser estimates what the property will be worth once the ADU is finished.

Fannie Mae’s HomeStyle Renovation program allows renovation costs of up to 75% of the as-completed value, and an ADU is an eligible improvement. Freddie Mac offers a similar program called CHOICERenovation. On the FHA side, a Standard 203(k) can fund an ADU added to an existing structure, such as a garage conversion or an attached addition, though it is not designed for a new freestanding unit in the backyard. Our FHA home loan page covers the FHA side in more detail.

With any renovation loan, construction funds are held in escrow and released in draws as work is completed and inspected. The lender also reviews the contractor, the bid, and the plans before closing, so it helps to have those lined up early.

ADU Construction Loan

An ADU construction loan is built specifically for ground-up projects. Some are construction-only loans, paid off by a separate mortgage once the unit is complete. Others are construction-to-permanent loans that convert into a regular mortgage after the build, with a single closing. During construction, payments are often interest-only on the amount drawn so far.

Many ADU construction loans are also sized on after-completion value, and some come from lenders who keep the loans in their own portfolio rather than selling them, which can allow more flexibility on larger or higher-value projects. In the East Bay, where home values often push loan amounts past conforming limits, that flexibility matters. Our guide to jumbo loans in California explains where those limits sit.

Paying Cash or Combining Sources

Some homeowners pay for design and permits out of savings and finance only the construction, or use a HELOC for a smaller first phase and refinance later. There is no single right structure. The goal is to match the funding to the timing of the bills, so money is available when the contractor needs it.

Here is how the main options line up side by side.

For many East Bay homeowners, the gap between current equity and the total project cost is the main hurdle. A finished ADU typically adds value to the property, and loans based on after-completion value let that future value count toward the loan amount.

Worth flagging: the appraiser has to support that value with comparable sales, ideally nearby homes that also have ADUs. In neighborhoods where those sales are still limited, the estimate can come in more conservatively than expected. Building some cushion into the budget, and giving the appraiser detailed plans and a complete contractor bid, helps keep the numbers on track.

One more number to plan for is property tax. The new unit is assessed and added to the property’s value for tax purposes, while the existing home keeps its current assessed value. Lenders factor the projected increase into the monthly payment when qualifying.

The CalHFA ADU grant reimbursed up to $40,000 in pre-development costs, such as plans, permits, soil tests, and impact fees, for eligible homeowners. According to CalHFA, the latest round of funding was fully allocated on December 28, 2023, and as of this writing no new round has been announced.

CalHFA has also warned homeowners about people offering to help secure an ADU grant while no funding is available. The official program page at calhfa.ca.gov/adu is the place to check for any future round. Some cities and counties run their own ADU incentives from time to time, so it is worth asking the local planning department during the permit process.

Underwriting looks much like any other mortgage: credit, income, assets, and debt-to-income ratio. On top of that, an ADU loan usually calls for a few project documents:

• Plans and permits, either approved or close to final
• A detailed contractor bid and signed construction contract
• The contractor’s license and insurance information
• A project budget that includes a contingency reserve

Some programs also allow a portion of the projected ADU rent to count toward qualifying income. Those rules are specific to each loan type, and our post on ADU rental income for FHA loans walks through how they work.

Can a junior ADU be financed the same way as a detached ADU? Yes, the same loan types apply. Because a junior ADU usually costs considerably less, a HELOC or a modest cash-out refinance often covers it without the extra steps of a renovation or construction loan.

Can ADU costs be included when buying a home? Often, yes. Renovation loans such as HomeStyle and the FHA 203(k) can be used on a purchase, combining the home price and the project budget into one mortgage, within each program’s limits.

Does building an ADU raise property taxes? It adds to them. The value of the new construction is added to the assessment, but the existing home is not reassessed because of the ADU.

ADU projects have a lot of moving parts, and the financing works best when it is planned alongside the design and permits rather than after. We start by listening to what the project is, where the current mortgage stands, and what the finished unit is meant to do. From there, we draw on our access to a wide range of lenders to find the best-suited option, from HELOCs to renovation loans and ADU construction loans.

Our office is in Pleasant Hill, and we work with homeowners throughout the East Bay and across California. To talk through an ADU project, call (925) 478-8630 or reach out here.

Mike Trejo is the Broker/Owner of Bridgepoint Funding, a residential mortgage brokerage based in Pleasant Hill, California. With more than 20 years of experience in the mortgage industry, Mike has helped thousands of borrowers navigate the home loan process, including many healthcare professionals and travel nurses with non-traditional income profiles.

Mike founded Bridgepoint Funding in 2006 and is consistently ranked among the Top 1% of Mortgage Loan Originators nationwide. Reach out at (925) 478-8630 or visit bpfund.com.