ADU vs JADU vs SB 9: Comparing Costs of Every Path to a Second Unit

Three Paths, Confusing Rules: Why Altadena Homeowners Can't Compare Costs

California hands homeowners three different pathways to a second unit — an ADU, a JADU, or an SB 9 lot split — and each one carries wildly different costs, fees, and fine print. Type "SB 9 cost" into a search engine and you'll get answers that contradict each other, partly because there are actually two different laws called SB 9.

The SB 9 most homeowners ask about is the 2021 law that allows splitting a single-family lot into two parcels, or building up to four homes on one lot. That's the pathway with the eye-opening price tag: engineering estimates put planning alone at $40,000–$70,000 — before a single wall goes up. Full-service projects typically land at $60,000–$70,000, and that range can exclude utility extensions, grading, and legal work.

The other SB 9 was signed October 10, 2025. That bill, sponsored by Senator Jesse Arreguín, lets the state's housing department void local ADU ordinances that violate state law, with state ADU standards applying until a city fixes its rules. It has nothing to do with lot splits — but because both share a name, search results blend them, and homeowners end up comparing costs from two unrelated laws.

The three pathways sit in completely different cost universes. A JADU — capped at 500 square feet — owes no impact fees or school fees, and as of January 1, 2026, no longer requires owner occupancy if it has its own bathroom. An ADU under 750 square feet skips development impact fees entirely. An SB 9 lot split, by contrast, starts at roughly the price of a small ADU build before construction even begins.

The market has already voted on this comparison. In SB 9's first year, Terner Center research found activity "limited or non-existent" across 13 studied jurisdictions. Los Angeles logged 211 SB 9 unit applications with only 38 approved, and 28 lot split applications with zero approved. Planners concluded that prescriptive state ADU laws are simply pushing homeowners toward ADUs instead.

What makes comparison genuinely hard is that the rulebook changes nearly every year:

• JADU owner-occupancy requirements relaxed as of January 1, 2026 for units with their own bathroom
• A single lot can now combine a converted ADU, a JADU, and a detached ADU
• Cities must rule on permit completeness within 15 business days and decide disputes within 60 business days
• Both ADUs and JADUs must be rented for terms longer than 30 days — no short-term rentals

One engineering firm puts the risk bluntly: the most expensive SB 9 project is often the one where a homeowner spends money in the wrong order. Feasibility should come before design, and design before any entitlement spending. That's why Altadena ADU Builder starts every homeowner with a feasibility-first conversation rather than a construction quote — the cheapest mistake is the one you never make. If you're weighing a backyard unit against a lot split, our free ADU feasibility consultation (with a cost-quiz concept render) is the low-cost first step before committing to any pathway.

The Hard Numbers: What Each Pathway Actually Costs Before Construction

When homeowners ask "what does a second unit cost," the honest answer starts before a single foundation is poured. The clearest hard number in this comparison belongs to SB 9: a lot split carries $40,000–$70,000 in planning-only costs before construction even begins, covering feasibility studies, surveying, engineering, agency review, and recordation. Full-service projects typically land at the higher end of that range, and the figure can exclude major utility extensions, unusual grading, lender and title work, and financing costs, according to detailed SB 9 cost analysis.

Contrast that with the fee picture for ADUs and JADUs, where California law works in the opposite direction. Units at or below 750 square feet are exempt from development impact fees entirely, and JADUs — capped at 500 square feet — dodge both impact fees and school fees, per legal analysis of California's 2025 ADU legislation.

It's worth being precise about what these numbers do and don't tell you. ADU and JADU construction costs vary by project — size, site conditions, and design choices all matter — and unlike SB 9's planning range, they aren't fixed regulatory figures anyone can quote in advance. What state law does fix is the regulatory and fee burden, and that's where the pathways genuinely diverge:

• SB 9 lot split: $40,000–$70,000 in pre-construction planning costs, with more possible depending on the property.
• ADU up to 750 sq ft: no development impact fees.
• JADU up to 500 sq ft: no impact fees and no school fees — the lightest regulatory load of the three.
• All pathways: construction budgets are project-specific, not set by statute.

The market has already voted on this cost structure. UC Berkeley's Terner Center found SB 9 uptake "limited or non-existent" across 13 jurisdictions in its first year — Los Angeles logged 211 unit applications with only 38 approved, and 28 lot split applications with zero approvals. Planners pointed to California's more prescriptive ADU laws, with their limited fees and larger allowed units, as the reason homeowners keep choosing ADUs over lot splits.

Riechers Engineering puts it bluntly: "the most expensive version of an SB9 project is often the one where a homeowner spends money in the wrong order." That's why feasibility should always come before design spending, regardless of which pathway you're weighing. It's the same principle we apply at Altadena ADU Builder — a free feasibility consultation with a cost-quiz concept render costs you nothing and tells you whether your Altadena property can actually create enough value to justify the process before you commit real money to any path.

Why Almost Nobody Uses SB 9: The Los Angeles Data Homeowners Should See

When California lawmakers passed SB 9 in 2021, they projected that more than 700,000 homes could become newly feasible on single-family lots statewide. The first-year reality told a very different story — and the numbers out of Los Angeles are the clearest evidence homeowners should see before betting on a lot split.

According to Terner Center research at UC Berkeley, Los Angeles logged 211 SB 9 unit applications in the law's first year, with only 38 approved. Lot splits fared worse: 28 applications, zero approved. San Diego, a city that permitted 871 ADUs in 2021, recorded just 7 SB 9 unit applications — none approved.

Why the gap? The planners Terner interviewed pointed to a simple explanation: state ADU law is more prescriptive, so homeowners route around SB 9 entirely. The practical advantages stack up quickly:

• Limited impact fees — ADUs of 750 square feet or less are exempt from development impact fees, while SB 9 projects face full fee exposure.
• No owner-occupancy requirement for most ADUs, unlike SB 9's residency rules.
• Larger allowed units — cities must permit ADUs of 850 to 1,000 square feet, and pre-approved plan programs speed up approvals.

The cost math reinforces the pattern. An engineering estimate of SB 9 lot split costs puts planning alone at $40,000–$70,000 — surveys, feasibility studies, agency review — before a single stud is framed. As one engineer bluntly warned, the most expensive SB 9 project is often the one where a homeowner spends money in the wrong order.

Meanwhile, ADUs kept gaining ground. Terner found that within six years of the state's ADU reforms, accessory dwelling units accounted for nearly a fifth of all new units permitted statewide. When one pathway offers fee exemptions and predictable rules, and the other offers six-figure planning risk and near-zero approval rates in LA, the market's verdict is not surprising.

For Altadena homeowners, the takeaway is straightforward: run the numbers on your actual parcel before committing to any pathway. That's why Altadena ADU Builder starts with a free feasibility consultation — a cost-quiz and concept render that shows whether an ADU, JADU, or something more ambitious makes sense on your lot. If a lot split still pencils out after that, you'll know. Most of the time, it won't.

2026 Rule Changes That Shift the Math: JADUs Get Cheaper, Stacks Get Real

If you priced out a second unit in 2024 and shelved the idea, it's worth pulling those notes back out. The rules changed on January 1, 2026, and several of the changes directly lower the cost or risk of the JADU and ADU paths.

The biggest shift involves JADUs. Under AB 1154, the owner-occupancy requirement now applies only when a JADU shares a bathroom with the main house. Build a JADU with its own bathroom — still capped at 500 square feet — and you can rent it without living on the property. That removes a constraint that pushed many Altadena investors toward full ADUs, and it stacks with the existing JADU advantage of no impact fees and no school fees.

The permitting process also gained teeth. Cities must now determine whether your application is complete within 15 business days, and they cannot introduce new requirements after resubmission. A 60-day approval shot clock applies to final decisions, including coastal permits, per current regulatory summaries. For homeowners who've heard horror stories about applications languishing for months, these codified timelines change the risk calculus.

The most consequential change for maximizing value: a single-family lot can now combine a converted ADU, a JADU, and a detached ADU — what some describe as "four ADUs" on one parcel as of 2026. But there's a catch worth flagging early:

• AB 956 takes effect January 1, 2027 and creates a real tradeoff: local agencies are not required to approve a JADU where two detached ADUs have already been built.
• Both ADUs and JADUs must now be rented for terms longer than 30 days — short-term rentals are off the table.
• Fee exemptions still favor smaller units: no development impact fees on ADUs of 750 square feet or less, and no impact or school fees on JADUs.

That AB 956 caveat matters for sequencing. If you're planning multiple units on an Altadena lot, building the JADU first — or at least locking it into your plans before adding a second detached ADU — may preserve options that disappear in 2027. The order in which you build now affects what you're allowed to build at all.

One clarification before you search further: the "SB 9" signed in October 2025 is a new ADU-ordinance enforcement bill, not the 2021 lot-split law. Per California YIMBY, the new bill lets HCD void local ADU ordinances that violate state law — which means noncompliant local rules are on shorter notice than they used to be.

These rule changes are exactly why Altadena ADU Builder recommends a feasibility-first approach: a free consultation to map which unit combinations your lot supports, before design spending begins. For Eaton Fire rebuild households weighing a JADU-plus-ADU strategy against the 2027 deadline, that sequencing question is the one to answer first.

Spend in the Right Order: The Feasibility-First Step Before Choosing a Path

For Altadena homeowners weighing a second unit, the costliest mistake often happens before construction ever starts. As Riechers Engineering warns, "the most expensive version of an SB9 project is often the one where a homeowner spends money in the wrong order" — meaning design fees paid before the property's eligibility is even confirmed.

That warning applies to all three pathways, not just lot splits. An SB 9 lot split carries $40,000–$70,000 in planning-only costs — feasibility, survey, engineering, agency review, and recordation — before a single board is nailed. And the payoff is uncertain: in Los Angeles, 211 SB 9 unit applications produced just 38 approvals, while all 28 lot split applications were denied, according to Terner Center research.

The smarter sequence costs almost nothing upfront. Before committing to any path, work through three checks in order:

• Confirm property eligibility. SB 9 requires a single-family parcel meeting specific conditions; an ADU or JADU is far more broadly permitted, with cities required to allow units of 850–1,000 square feet.
• Verify fee exemptions. ADUs of 750 square feet or less pay no development impact fees, and JADUs capped at 500 square feet avoid both impact and school fees entirely.
• Match the pathway to the lot. Smaller lots and tighter budgets usually favor a JADU or detached ADU; a lot split only makes sense if the property can create enough value to justify the process.

The order matters because the rules shifted for 2026. Under new state legislation, a JADU with its own bathroom no longer carries an owner-occupancy requirement — a change that can flip the math for Altadena homeowners who previously ruled the option out. Rental terms must still exceed 30 days on all second units.

One caution as you research: timelines and permit outcomes can't be guaranteed. State law sets procedural shot clocks — a 15-business-day completeness review and 60-day approval window — but actual results in unincorporated LA County depend on your specific parcel, plans, and review cycle. Any contractor or consultant promising a guaranteed approval date should raise a red flag.

The natural first step is a free ADU feasibility consultation with Altadena ADU Builder, which includes a cost-quiz concept render of what your property could support. It's the low-cost way to answer the eligibility and pathway questions before spending on surveys, designs, or a split application — so your money goes where it actually moves the project forward. If you move ahead with construction, JMB Builders LLC (CSLB #1103479), an Altadena-based licensed general contractor and featured builder in a disclosed paid placement, builds ADUs and custom homes locally, including Eaton Fire rebuilds.