California SB 1117 Would Fix ADU Impact Fee Structure for Homes Over 750 Square Feet

California homeowners trying to build a backyard unit are being hit with fees that don’t add up — and SB 1117 may be the bill that finally fixes the math. As Sacramento advances another round of housing legislation, millions of property owners across all 58 counties are asking the same question: why does building a slightly larger home on your own land trigger a financial penalty that punishes common sense?

California’s Housing Crisis Is Real — and ADUs Are Carrying More Weight Than Ever

The numbers don’t lie. In 2025, California’s housing shortage was still estimated at 3 million housing units, a deficit decades in the making and still growing. California increased its total housing stock by only 0.84% in 2024 — just 125,000 units — and ADUs made up about one-fifth of those units, according to California Department of Finance data. WikipediaArcaMax Publishing

One-fifth of all new housing in the most expensive state in the nation is coming from backyard cottages and converted garages. That is not a footnote — that is a policy result. And yet the regulatory framework surrounding ADU construction still contains a fee structure that discourages homeowners from building units large enough to be genuinely livable. ADUs account for about 20% of the state’s new housing, and in contrast to large apartment buildings or expanded suburbs, they provide a “surgical” path to new housing using existing land, sewers, water, and other infrastructure. DNM Architecture

ADUs now represent roughly 20% of California’s new housing supply. The question no one in Sacramento wants to answer plainly: why are we still taxing homeowners who try to build them?

What Is the Fee Problem SB 1117 Is Trying to Fix?

The existing fee structure creates a cliff — and most homeowners don’t see it coming until they’re already in the permitting process. Under current California law, ADUs with 750 square feet of interior livable space or less are exempt from impact fees charged by local agencies, special districts, and water corporations. But if an ADU exceeds 750 square feet, impact fees are charged proportionately in relation to the square footage of the ADU compared to the primary dwelling unit. CA

The problem is in how that proportionality is calculated. Under the current formula, once an ADU crosses the 750-square-foot threshold, fees are assessed against the entire unit — not just the portion above the limit. A homeowner building an 800-square-foot unit pays fees on all 800 square feet. That’s a sudden, steep increase triggered by adding just 51 extra square feet. It punishes efficiency.

SB 1117 fixes this by switching to a marginal fee structure. Instead of charging fees based on the entire ADU’s size once it exceeds 750 square feet, local governments can charge proportionally only for the square footage above 750 square feet. This is the same logic behind marginal tax rates — you pay more as you earn more, but you don’t suddenly owe more on everything you already had. California YIMBY

“Why should a California homeowner who builds a backyard unit one inch above a government threshold owe fees on the entire structure? That’s not proportionality — that’s a penalty.”

Where Does SB 1117 Stand Right Now?

This bill is moving — and moving fast, with near-unanimous support. SB 1117 was introduced in the Senate on February 17, 2026. It passed the Senate Housing Committee on April 7, 2026, and the Senate Local Government Committee on April 22, 2026. It passed the full Senate floor on May 18, 2026, by a vote of 37-0. California YIMBY

As of June 10, 2026, the bill cleared the Assembly Housing Committee with a 10-0 vote and was re-referred to the Assembly Local Government Committee. It is authored by Senator Cervantes and applies to all cities in California, including charter cities, which the bill’s findings classify as a matter of statewide concern. Digital Democracy

A 37-0 Senate vote. That is not a partisan bill. That is a consensus correction to a policy error — and its broad support should tell residents something about how obviously broken the current fee formula is.

Are Homeowners and Local Governments Actually Aligned on This?

The 37-0 vote is telling, but local governments have historically used fee structures as a tool to limit housing growth they don’t want. Cities have actively undermined the goals of state housing laws through restrictive ordinances and creative interpretations, according to YIMBY Law. The offenders span the political spectrum, from slow-growth liberal Marin County communities to conservative Orange County cities that used “not in my backyard” arguments in lawsuits against state housing laws. Governing

If even California’s most resistant local governments couldn’t muster a single “no” vote in the Senate, it’s worth asking: what took so long?

The answer may lie in the incremental nature of ADU reform itself. California has layered bill upon bill since 2016 — each session patching holes the previous session left open. The California Legislature enacted four new bills making further changes to ADU and JADU laws in 2025 alone, with most taking effect January 1, 2026. SB 1117 is part of that same corrective pattern: not a revolution, but a repair. Bwslaw

What Do Supporters of This Policy Actually Believe?

Supporters of the current impact fee system argue that local governments need revenue to fund public services — roads, parks, schools, utilities — and that new housing, even modest ADUs, creates real demand on infrastructure. They contend that waiving or reducing fees shifts the cost burden onto existing residents and taxpayers who already paid into those systems. Some local finance officials argue that a marginal fee approach, while seemingly fair, reduces the revenue available for infrastructure maintenance in ways that are difficult to forecast or offset.

These are legitimate concerns worth taking seriously. Infrastructure costs are real, and someone has to pay for them. But the current cliff-based formula doesn’t solve that problem — it just imposes a sudden penalty at an arbitrary threshold. Terner Center research has demonstrated that overly large fees can influence the cost to construct both market-rate and affordable housing. A marginal fee model doesn’t eliminate revenue to local governments — it restructures how that revenue is collected, more fairly, in proportion to what is actually being built. Charging fees on only the square footage above 750 square feet still generates revenue. It just stops punishing homeowners for building one extra room. Terner Center

Who Is Really Paying the Price for the Current System?

The answer is straightforward: middle-class California homeowners who want to house an aging parent, generate rental income, or help a grown child afford to stay in the state. These are not developers. These are residents trying to use their own property responsibly — and a fee cliff is one of the hidden barriers standing between them and a finished unit.

ADU production was up 14.3% in 2024 over the previous year, and in 2023 ADU production had grown by 10%. Demand is rising. But demand constrained by unpredictable fees is demand that never becomes supply. Every homeowner who scraps an 850-square-foot design and shrinks it to 749 square feet to avoid the fee cliff is a unit that got smaller — not because the family needed less space, but because the government made the alternative too expensive. ArcaMax Publishing

If a homeowner in your neighborhood wanted to add a backyard unit to house their mother-in-law — and the government’s fee formula made it financially impossible to build one big enough to actually live in — would that be a housing policy, or a housing obstacle?

Key Questions This Story Raises

• Why has California’s impact fee formula used a cliff-based structure for years when a marginal model is both more equitable and less disruptive to housing supply?
• If SB 1117 passed the full California Senate 37-0, why is this bill not receiving greater public attention — and who benefits from keeping homeowners in the dark about fee reform?
• What other hidden cost triggers in California’s ADU permitting process are still discouraging homeowners from building units that could ease the state’s 3-million-home shortage?

What Comes Next — and What You Can Do

SB 1117 is now in the Assembly Local Government Committee. It still needs to clear the full Assembly and, ultimately, Governor Newsom’s desk. Given its unanimous Senate passage, the path forward looks clear — but housing bills with strong Senate support have stalled in Assembly committees before.

California YIMBY, one of the bill’s prominent supporters, describes SB 1117 as a measure that will make it less costly to build ADUs by removing what it characterizes as an arbitrary financial penalty that many jurisdictions impose on ADUs over 750 square feet. California YIMBY

Californians who want to see this bill signed into law can contact their Assembly representatives directly and ask them to vote yes when SB 1117 comes to the floor. A petition in support of the bill is circulating — find it linked in our profile. The window to make your voice heard is now.

The real question isn’t whether SB 1117 is good policy — the 37-0 Senate vote already answered that. The question is whether enough Californians will demand their Assembly members finish the job.

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Still have questions about how SB 1117 affects your ADU plans? Let us know in the comments — and contact your Assembly representative to make your voice count.