ADUs in San Diego: Rules, Costs, and the Rental Math

Key takeaways

• California's ADU law (Gov. Code §§66310–66342) requires ministerial, staff-level approval of a qualifying ADU or JADU application, generally within 60 days, with no public hearing and no CEQA review.
• The state sets a floor, not a ceiling, on size: a city or county cannot cap a detached ADU below 850 square feet of interior livable space (1,000 if it has more than one bedroom), cannot require rear/side setbacks greater than 4 feet for new construction, and cannot require more than one parking space per unit — or any parking at all within a half-mile of transit, in a historic district, or for a conversion ADU.
• As of January 1, 2024, state law permanently bars local agencies from imposing an owner-occupancy requirement on an ADU. A junior ADU (JADU) is treated differently: if it shares a bathroom with the main house, the owner must still occupy either the JADU or the primary residence, with limited exceptions.
• Impact fees are waived entirely for an ADU under 750 square feet of interior livable space, and for any JADU; a larger ADU can only be charged a fee proportional to its floor area against the primary home's.
• The City of San Diego runs an ADU Bonus Program that lets a deed-restricted affordable ADU unlock an additional market-rate “bonus” ADU, with no cap on the total inside a Transit Priority Area. The City adopted a package of reforms to this program in 2026 — verify the current rules before you design to it.
• Building an ADU does not reassess your whole property under Proposition 13. Only the new construction (the ADU itself) gets a new base-year value added to your existing assessed value; the rest of the home keeps its base-year value and its 2%-a-year cap.
• Fannie Mae's Selling Guide (B3-3.1-08, updated for ADU income in late 2025) lets a lender count rental income from one ADU toward loan qualification, generally at 75% of the lease or market-rent figure to account for vacancy and expenses — the same rule sounding's move-up mode runs against a specific address.

The state framework: what cities can no longer control

State ADU law has been renumbered and amended repeatedly since the current framework's predecessor (SB 1069, AB 2299, and later bills) took effect. The current version lives at Government Code §§66310–66342, and HCD publishes a plain-language Housing Law Fact Sheet and a longer ADU Handbook that local planning departments are expected to follow. The fact sheet spells out what a city or county's ADU ordinance is not allowed to do:

A jurisdiction can still apply objective design standards — a specific setback distance, a specific height number — but not subjective ones like “consistent with neighborhood character.” Local governments can restrict ADU rentals to terms of more than 30 days, which is aimed at blocking short-term rental use rather than long-term tenants. A JADU is a smaller category: a unit built entirely inside the walls of an existing or proposed single-family home (including a garage conversion), capped at 500 square feet, limited to one per lot, and it can share a bathroom with the main house — but if it does, the owner must occupy the JADU or the primary home, one of the few owner-occupancy rules state law still allows.

Recent legislative activity keeps landing in this space: 2025's SB 543 clarified that floor-area limits apply to interior livable space and set a 15-business-day application-completeness clock, and 2024's SB 1211 expanded the number of detached ADUs allowed on multifamily lots to as many as eight. HCD also actively reviews local ordinances for compliance and has issued findings against several California cities in the past year for ADU rules that fell short of the state floor. None of this is exotic policy trivia — it means the state framework changes more often than most local ordinances do, and the HCD ADU Handbook is the document to check first.

City of San Diego: the ADU Bonus Program

Inside city limits, a single-family lot with an existing or proposed dwelling can generally add one ADU and one JADU. A multifamily lot has more room: state and city rules together allow up to two new detached ADUs, converting existing habitable space into ADUs up to roughly a quarter of the lot's existing unit count, and converting non-habitable space (basements, storage) into ADUs without a specific numeric cap.

San Diego's notable local layer on top of the state floor is its ADU Bonus Program: deed-restrict an ADU as affordable to very-low-, low-, or moderate-income households for 15 years, and the city permits an additional market-rate “bonus” ADU on the same lot. Inside a designated Transit Priority Area (generally a half-mile of a major transit stop), the program removes the numeric cap on total ADUs entirely, limited practically by what fits on the lot; outside a TPA the bonus is capped and, following a 2026 reform package, requires an off-street parking space per bonus and affordable unit.

The City Council has been actively amending this program — a comprehensive reform package moved through committee and adoption in 2026, touching unit counts, minimum unit size, fire and evacuation-route standards, and new community-enhancement fees. Because this program sits on top of (not instead of) the state framework and keeps getting revised, confirm the current version with San Diego's Development Services / Planning Department before assuming a specific bonus ratio or fee applies to your lot.

County of San Diego: the unincorporated rules

Outside city limits, the County's Planning & Development Services (PDS) administers its own ADU ordinance, which — like every California jurisdiction's — has to sit on top of the same statewide floor described above. A residential parcel with an existing or proposed single-family dwelling can generally add one ADU and one JADU (JADU capped at 500 square feet, as under state law); some County materials describe higher size allowances for a detached unit than the statewide 850/1,000 sq ft floor, since a local jurisdiction is always free to allow more than the state minimum, just not less. Confirm the exact detached-unit cap and setback standard currently in the County's ADU ordinance and handbook before designing to a specific number.

The County has also kept its ordinance moving: on March 4, 2026, the Board of Supervisors unanimously adopted an amendment implementing Assembly Bill 1033, which opens a path for the separate sale of an ADU from the primary home in unincorporated communities, through a condominium-conversion-style process. That is a meaningful change to how an ADU can be financed and exited — worth a direct conversation with PDS if it's relevant to your plans, since implementation details (which zones qualify, what the conversion process actually requires) are the kind of specifics that change fastest.

The tax angle: Prop 13 and new construction

This is the piece most homeowners get wrong, and it cuts in the homeowner's favor. Proposition 13 caps the annual growth of a property's assessed value at 2% and resets that base only on a change of ownership or on new construction. Adding an ADU is new construction — but California's Board of Equalization is explicit that only the new construction itself is reassessed, not the whole parcel. The assessor estimates the added market value the ADU itself contributes, assigns that increment its own new base-year value, and the pre-existing home keeps its original base-year value and its 2% annual cap untouched.

Mechanically, the county assessor typically issues a supplemental assessment and a supplemental tax bill for the ADU's added value once construction is finished and recorded, separate from your regular annual bill — the same mechanism that produces a supplemental bill after a purchase, covered in more detail in the supplemental tax bill guide. If your home already sits inside a Community Facilities District, adding an ADU doesn't change that fixed Mello-Roos special-tax line either way — that charge is set by the district's formula, not by your home's assessed value; see What Is Mello-Roos? for how that separate charge works.

The rental math: broad cost ranges, and whether the rent covers it

Build cost for an ADU varies enormously by size, finish level, foundation type, and site conditions (grading, utility hookups, sewer/septic, slope). As a very broad, non-binding planning range: a junior ADU built inside existing walls tends to run at the low end because it reuses structure and roofline, a garage conversion sits above that, and a ground-up detached unit costs the most per square foot because it needs its own foundation, roof, and utility connections. Treat any number you see quoted — including any single figure floating around online — as directional at best; get a written estimate from a licensed contractor for your actual lot before you budget against a specific figure.

Whether an ADU is worth building usually comes down to whether the rent it can command covers its share of the debt used to build it, plus the added property tax from its new base-year value, plus incremental insurance and utility cost. Two ways that shows up:

• As a standalone rental. Compare the ADU's achievable market rent (a comparable rent for its size and location) against the monthly cost of whatever financing built it — a cash-out refinance, HELOC, or construction loan. If the rent doesn't clear that monthly cost plus the added tax and insurance load, it's a lifestyle or resale-value decision, not a cash-flow-positive one.
• As qualifying income when you buy or refinance. Fannie Mae's updated Selling Guide (B3-3.1-08) lets a lender count rental income from a single ADU toward loan qualification on a one-unit primary residence, generally at 75% of the lease amount or an appraiser's market-rent opinion — the discount is there to cover vacancy and operating costs the lender assumes you'll bear. That income is capped at a share of total qualifying income and generally applies to a purchase or a limited cash-out refinance, not every loan type. This is the same 75% rent-qualification math sounding's move-up mode runs against a specific address, letting you compare keeping a current home (with its ADU) as a rental against selling it outright. If you're weighing the broader rent-vs-buy tradeoff first, the rent vs. buy guide and the income-to-buy guide work that math for a primary purchase.

Common questions

Do I need to live on the property to build an ADU in San Diego?

No, for an ADU itself — state law has permanently barred an owner-occupancy requirement since January 1, 2024. A JADU is the exception: if it shares a bathroom with the main house, the owner must occupy either the JADU or the primary residence, with limited exceptions.

How big can my ADU be?

State law sets a floor a local ordinance can't go below — 850 square feet (1,000 with more than one bedroom) of interior livable space — but a city or county is free to allow more. Confirm the current maximum with the City's Development Services Department or County PDS for your specific zone.

Will building an ADU trigger a full property tax reassessment?

No. Only the ADU's own new construction value gets a new base-year value added to your bill. Your existing home keeps its original Prop 13 base year and its 2% annual cap. Expect a separate supplemental tax bill for the ADU's added value once it's finished.

Do I have to pay impact fees?

Not if the ADU's interior livable space is under 750 square feet, and never for a JADU. A larger ADU can only be charged a fee proportional to its floor area relative to the primary home.

What is San Diego's ADU Bonus Program?

A City program that lets a deed-restricted affordable ADU unlock an additional market-rate bonus ADU on the same lot, uncapped in number inside a Transit Priority Area. The City Council adopted a reform package to this program in 2026, so confirm the current ratios, parking rules, and fees with the City before designing to it.

Can I sell my ADU separately from the main house?

Generally no under the standard ownership model, but the County of San Diego adopted an AB 1033 implementation ordinance in March 2026 that opens a condominium-conversion path for separate sale of an ADU in unincorporated areas. Confirm eligibility and process with County PDS.

Does adding an ADU help me qualify for a bigger loan?

It can. Fannie Mae's updated guidance lets a lender count roughly 75% of an ADU's rental income toward qualifying income on certain loan types, subject to caps — the same rule behind sounding's move-up mode math.

Run the numbers for your address

Every figure above is a statewide floor, a stated local program rule, or a broad planning range — not a quote for your specific lot, zone, or contractor bid. Generate a free San Diego property report for the parcel-specific facts (assessed value, tax rate area, zoning context) an ADU decision starts from, or run the rental side directly in move-up mode, which applies the same 75% rent-qualification rule a lender uses.

Related reading

• Rent vs. Buy in San Diego
• How Much Income It Takes to Buy a $1M Home in San Diego
• Prop 19 for Move-Up Buyers
• What Is Mello-Roos?
• The Supplemental Tax Bill Surprise