California ADU Financing and SB 9 Lot Splits: Lender Underwriting Challenges and Legal Issues

California's SB 9, which took effect January 1, 2022, was designed to meaningfully increase housing density by allowing homeowners in single-family residential zones to split their lot into two parcels and develop up to two units on each — theoretically quadrupling the number of units allowable on a standard residential lot. Combined with California's existing accessory dwelling unit statutes (Government Code section 65852.2), which separately authorize ADUs and junior ADUs on residential parcels, the legislature created a legal framework that allows substantial densification without going through discretionary planning approval.

The implementation reality has been considerably messier. Cities have been slow to update their ministerial review processes, and many homeowners who obtained approved lot splits under SB 9 have discovered that their local jurisdiction does not yet have a clear process for issuing building permits on the newly created second parcel. More significantly for real estate attorneys and their clients, the financing market for SB 9 lots and ADU construction has developed unevenly.

For ADU construction financing, the core challenge is that most conventional residential mortgage lenders (Fannie Mae and Freddie Mac conforming loan guidelines) treat ADU income as accessory income — it does not count toward the primary borrower's qualifying income for the underlying mortgage at full value, and it cannot be used to repay a standalone construction loan until the unit is completed and a tenant is in place. This means that homeowners seeking to build ADUs frequently cannot access conventional financing to cover construction costs, and must turn to home equity lines of credit, construction loans, or specialized ADU lenders — all of which carry higher rates and fees than conforming mortgages.

SB 9 lot splits create an additional title complexity. When a lot is split under SB 9, the new parcel must be conveyed separately — the two resulting parcels cannot be held under a single mortgage instrument with a conventional lender. This means the lot split transaction itself requires new title work, new surveys, and potentially new financing instruments. A homeowner who splits their lot and intends to sell the new parcel faces the challenge that the new parcel is typically an unimproved vacant lot in an area zoned residential rather than commercial — the comparable sales are essentially nonexistent, making appraisal of the new parcel for conventional purchase financing purposes extremely difficult.

Several California-focused lenders have developed ADU-specific loan products that address some of these constraints — notably CalHFA's ADU Grant Program (which provides grants of up to $40,000 for ADU construction costs) and its ADU loan program, which uses projected rental income in underwriting. Additionally, some credit unions and community banks in California have developed portfolio loan products for ADU financing that are not subject to Fannie/Freddie guidelines.

For real estate attorneys advising clients in ADU or SB 9 transactions, the practical priority is early coordination between the client, a specialized ADU lender or construction finance specialist, and the title company. Title companies have varying familiarity with SB 9 lot split procedures, and some have declined to insure lot split parcels pending further clarification of ministerial review records. Attorneys should identify title companies with confirmed SB 9 experience before the client commits to the transaction timeline.