How California ADU Law Forced San Marino To Change

Can you build an ADU in San Marino? Today, yes. This episode explains how California ADU law opened the door, what the rules now require, and how homeowners are using backyard homes to keep their families close in one of the most carefully preserved cities in Los Angeles County.

San Marino feels deliberately preserved, like everyone quietly agreed to keep it steady. That care is exactly what makes it such an instructive example of how state ADU law and thoughtful communities are learning to work together. We trace how California's reforms established a clear, objective path to approval, what written guidance from the California Department of Housing and Community Development clarified along the way, and how San Marino's own state-accepted housing plan now embraces accessory dwelling units as a meaningful part of meeting its RHNA housing goals. If terms like ministerial approval, ADU ordinance compliance, or HCD guidance have you wondering what they mean for your backyard, you will leave with a clearer map of the rules and how they protect both homeowners and the character of the community.

Most importantly, we get to what this means for your family. With San Marino home prices in multi-million-dollar territory, the biggest threat to family continuity is your kids getting priced out. We walk through two high-impact ADU strategies: a launch pad for adult children, and a house swap that supports aging in place while keeping grandkids close. We also cover the questions to bring to qualified tax and legal professionals around Proposition 13 and Proposition 19.

A well-designed ADU does not have to change the feel of a street. Done right, it preserves something more important: the ability for the next generation to stay.

If this hits home, subscribe, share the episode with a neighbor, and leave a review.

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Thinking about building an ADU in California? The questions we hear every day: Can we build one? What about permits? Who can we trust to do it right?

That's Acton ADU. For 35+ years we've designed and built high-quality accessory dwelling units—long before backyard homes were a trend, and we handle the whole process, permits included.

Serious about an ADU? Start with the right team. Visit ActonADU.com or call (408) 539-1908—and let us know you heard about us on the s

This episode of ADU Adventures: Building in Your Backyard was sponsored by Acton ADU—California's experts in high-quality, long-lasting accessory dwelling units (ADUs). With 35+ years of experience, they handle everything from design to permits to construction, making the whole process seamless for homeowners.

Thinking about building an ADU? Visit ActonADU.com or call (408) 539-1908 to get started.

Thanks for listening—see you next time!

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With more than 35 years of design-build experience, Acton ADU specializes exclusively in Accessory Dwelling Units (ADUs). Based in Campbell, California, we serve homeowners across more than 80 jurisdictions  throughout in the San Francisco Bay Area, the East Bay, and the San Gabriel Valley.

We design and build high-quality, traditionally site-built backyard homes, delivering turnkey ADU solutions that save time and money without sacrificing quality. As the home of the Build Ready ADU, we offer curated, pre-designed plans built around multigenerational living and long-term property value, with pre-approved options in select cities for faster timelines.

Why Homeowners Choose Acton ADU:

• More than 35 years of design-build expertise
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Whether you are building a mother-in-law suite, granny flat, or casita, Acton ADU is your trusted partner in creating a backyard home your family will rel...

Transcript

Why San Marino Matters

SPEAKER_00 0:31

Welcome back to ADU Adventures Building in Your Backyard. I'm James, and today's episode is a little different. Because today we're going to go somewhere that holds a special place in my family's heart. We're talking about San Marino, California. My wife grew up in San Marino. Her formative years happened there on those streets. She went to school there. She grew up under those old trees, and she experienced firsthand what makes that town feel different from almost anywhere else in Southern California. When she talks about her childhood, she isn't just talking about the house or describing the yard. She's describing a place that felt permanent, felt steady, like it had been arranged carefully a long time ago. And everyone had quietly agreed to keep it that way. And here's the thing, they kind of had. San Marino might be one of the most intentionally unchanged cities in California. And that's exactly why I want to talk about it today, because San Marino is also at the center of one of the most dramatic ADU stories in the state. A homeowner who took her city all the way to the California Supreme Court over a garage, a state housing agency putting formal findings on paper, and a quiet legal reality that means this famously change-resistant town is now committed on paper to hundreds of backyard homes. So here's where we're going. First, a little bit of the history because you can't understand San Marino's relationship with change unless you understand how the town was built and who lives there now. Second, the ADU fight told start to finish, so you can see exactly how the door opened. And third, the heart of it, what I believe ADUs actually mean for San Marino homeowners. It's about continuity. Keeping your family close on that ground your family worked so hard to hold. So if you own a home in San Marino, or in a town that's a little bit like it, this one's for you. So let's dive in. A city built to stay the same. To understand San Marino, you have

A City Designed To Stay Fixed

SPEAKER_00 2:37

to go back before it was a city, when it was a ranch land sitting at the foot of the San Gabriel Mountains. In the 1800s, a rancher named James DeBarth Shorb ran a large estate there, and he named it San Marino after the Maryland farm where he was raised. Then in nineteen oh three, a railroad magnate named Henry E. Huntington, one of the wealthiest men in California, bought the San Marino ranch from the Shorb family and made it his personal estate. The name Huntington sounds familiar, it should. The estate later became the Huntington Library, Art Museum, and the Botanical Gardens, one of the great cultural institutions in the country, and it still sits there right in the middle of town. In fact, the Huntington's own historical exhibition on the founding families is where much of this early history is documented. Then in nineteen thirteen, according to the City of San Marino's official history, the ranchos of Wilson, Patton, and Huntington, along with the smaller neighborhood ranchos, incorporated together as the city of San Marino. And here's a little detail I love. The city's first mayor was George S. Patton Sr., and yes, his son was that George S. Patton, the World War II general. That's the kind of history sitting quietly around San Marino. Now here is the part that explains everything that comes later. San Marino wasn't a town that grew up gradually and got zoned along the way. It was designed from its founding as a finished place. And the city is divided into zones by minimum lot size, and some of those minimums run past 30,000 square feet. San Marino is fewer than 13,000 people, less than four square miles, and has a zoning code that functioned like a preservation order for over a century. But while the zoning never changed, the people did completely. The San Marino my wife grew up in was already transforming into the San Marino of today, and today's San Marino is one of the most remarkable communities in America. The U.S. Census Bureau estimates roughly 69% of San Marino's residents are Asian American, the majority of them Chinese and Taiwanese descent. And nearly half of the city's residents were born outside of the United States. Starting in the 1980s, successful immigrant families arriving in the San Gabriel Valley set their sights on the region's most prestigious addresses, and San Marino was at the top of that list. These are families who didn't inherit San Reno. They crossed an ocean, built businesses and careers, and bought into San Marino deliberately, often specifically for the schools and the stability, as the accumulation of an enormous family effort. And here's why that matters so much for today's topic. For many of these families, multigenerational living isn't a trend or a compromise. It's the natural shape of the family, grandparents helping raise grandchildren, adult children staying close, extended family on the same property or the same street. The instinct that San Marino's founders built into the zoning, permanence, roots, generational thinking, is the same instinct today's San Marino families carry culturally. The town and its people agree on the goal. They've just been using different tools. Which brings us to the fight, the big fight, the big ADU fight. Because when California's ADU laws arrived, the city government and the state went to war over backyards. And the story of that fight is the story of why every San Marino homeowner now has an option their city spent years trying to take off the table. So here it is the fight over the backyards, San Marino's ADU story. Let me tell this in order because the sequence is everything. It's 2017. A San Marino homeowner named Cordelia Donnelly applied for a permit to build an ADU above her detached garage. Same footprint as the garage that was already standing. The

Who Lives Here Now And Why

SPEAKER_00 6:42

city denied her. Their reasons under the local ordinance at the time were that her lot was too small, her proposed unit was too big, and the garage sat too close to the main house. Donnelly didn't accept that. A property rights law firm called Pacific Legal Foundation took her case, arguing that California's state ADU law overrides local restrictions like the ones San Marino used. The case went up through the courts, and in 2020 it reached the California Supreme Court, which declined to review it. So in the narrow technical sense, the city won. It successfully denied one homeowner's backyard unit. But here's the twist, and it's the hinge of the whole episode. While that case was winding through the courts, Sacramento was rewriting the rulebook. A package of ADU reforms took effect on January 1st, 2020, and it eliminated the exact tools San Marino used against Donnelly. Minimum lot sizes for ADU eligibility gone statewide. The size and the separation restrictions she ran into directly targeted. The state's own ADU handbook lays out the current framework. ADU approval in California is ministerial, meaning if a project meets the objective written standards, the city must approve it. No hearings, no neighborhood veto, no planning commissioner's personal taste. In other words, San Marino won a fight over rules that stopped existing almost immediately afterward. Donnelly lost her case, but every San Marino homeowner after her inherited a completely different playing field. Did the city adapt gracefully though? Well, not exactly at first. In March 2024, San Marino adopted a brand new ADU ordinance presented as bringing the city into compliance. The California Department of Housing and Community Development, or the HCD, the state's housing agency, reviewed it and in written findings issued August 26, 2024, concluded that the new ordinance still didn't comply with state law. The findings,

The Garage Case And New ADU Laws

SPEAKER_00 9:26

which are public, called out height limits below the state minimums, design review language that left room for subjective judgment, and denial procedures that didn't follow the state's rules, which require a city denying an ADU to say in writing within 60 days exactly what's deficient and how to fix it. And the state gave San Marino 30 days to respond. Now, if the story ended here, you might conclude that any city can just stall forever. Well, it can't. And this is the part almost nobody outside of City Hall understands. Every California city is required by the state to plan for its share of new housing. That share is called the Regional Housing Needs Assessment. San Marino's assigned share of the 2021 through 2029 cycle is 397 units. And here's the thing about a city that's fully built out. A single family home with no vacant land. There's essentially only one way to get there, and that's backyard homes or ADUs. In fact, the San Marino Tribune reported numbers when the city's housing plan moved forward in 2024. Based on roughly 100 ADUs permitted between October 2021 and June 2024, the city's state accepted housing plan anticipated a total of 287 ADUs built in San Marino during this cycle. I'll say that again. San Marino's own adopted plan, the one that the state signed off on, projects 287 backyard homes by 2029. For a city of fewer than 13,000 people, that is a transformation, and it's one the city has legally committed to. Falling badly short isn't a free choice. It exposes the city to serious legal consequences under state housing law, which is precisely why the council approved the plan. And the tone at City Hall tells you everything about how we got here. At the November 2024 council meeting, where the housing plan advanced, one councilman compared the vote to being a kid forced to eat Brussels sprouts. Nobody at the table liked it, but everyone understood that refusing would cost the city far more. The community development director in the Tribune's report framed the result as complying with the state mandates while preserving as much of the city's character as possible. Meanwhile, and I find this genuinely remarkable given the history, the city has started leaning in. San Marino's City Council voted unanimously to waive its ADU application fee for a year on proposed ADUs over 800 square feet. The city that once fought a homeowner to the Supreme Court over a garage is now quietly making it cheaper to apply. So here's the oddest bottom line. And I say this as someone who genuinely loves this town. The question of whether ADUs are coming to San Marino has already been answered. The state law answered it. The courts closed the workarounds, and the city's own housing plan now depends on them. What remains is a little bit of friction. A building department with a reputation for scrutinizing every application, every setback, every design detail. The resistance can slow a project down. It can no longer stop one, though, which means the real questions left for San Marino homeowners are the good ones. What will these two hundred and eighty seven backyard homes be? Who designs them well enough to pass an exacting review the first time? Do they honor the street they sit on or cheapen it? And most importantly, who are they really for? That last question is where I want to spend the rest of this episode. Because I believe the most powerful use of an ADU in San Marino has almost nothing to do with what people fear, and almost everything to do with what this town and the families in it have always been about. The continuity math. Why staying is the new moving up. Let's talk about numbers for just a second because San Marino, well, the numbers tell a family story. According to Redfin, the median sale price in San Marino was $3.13 million as of May 2026, up 2.7% year over year. And homes.com runs the affordability math right on its San Marino page using the standard 28% rule with 20% down on a 30-year mortgage at recent rates. A buyer would need to earn roughly $640,000 a year to afford the median home there. Let that sink in for a second. $645,000 a year to buy into the town where your kids grew up. So here's the uncomfortable truth for San Marino families. And I mean this with respect. The greatest threat to the character of your family's life in San Marino is not an ADU in somebody's backyard. It's the fact that your own children, in most cases, will never be able to buy a home there. And for so many San Marino families, that lands with particular weight. If your family crossed an ocean to get here, if buying into San Marino was the summit of a generational effort, then the idea that the next generation gets priced right back out isn't just

State Findings And Housing Quotas

SPEAKER_00 14:59

a housing statistic. It undoes the climb. This is a town full of families who moved mountains to get in, just like my wife's family. And in no small part, it was for the schools. Great schools data notes that San Marino's four public schools carry an average rating of 10 out of 10. Families who are already there are holding something that is very close to impossible to replace, which brings me to what an ADO actually is in a town like San Marino. It's not about density, it's not an income play. It's a continuity tool. And for a community where multigenerational living is already the cultural norm, it isn't even a new idea. It's the family structure you already have, finally given to its own architecture. Picture the two most common versions. Version one, the launch pad. Your daughter finishes school, she's building a career, maybe starting a family of her own, buying in San Marino is out of reach, and renting nearby drains everything she's trying to save. A beautifully designed ADU on your property gives her a real home. Private, dignified, hers. And your grandkids grow up steps away in the same town, walking toward the same schools that shape their mother. Version two is the house swap. We see this a lot in the Bay Area too. This is the one I find the most powerful. At some point, the big house becomes a lot of house. Stairs, maintenance, rooms nobody opens. The traditional answer is to sell and downsize out of town. And just like that, decades of roots are gone. The ADU answer looks completely different. You build a single story home in your backyard designed from day one for aging in place. You move in. Your son or daughter's family moves into the main house. The grandkids walk to the same schools their parents walk to. Their grandparents are 30 feet away, not 30 minutes, not a plane flight away. Nobody leaves. The family holds its ground. Now, two financial realities you need to understand before you get excited. And please hear the disclaimer first. I am not a tax advisor, an attorney, or a financial planner, and uh California property tax law is genuinely complicated. So talk to a qualified professional before acting on anything I'm about to say, because this is just me and you right now. I'm just telling you my ideas. First, uh the good news. Under Proposition 13, the law that caps how fast California property taxes can grow, your property generally can't be reassessed just because time passes. Reassessment is triggered by a change of ownership or by new construction. Building an ADU counts as new construction, but here's the key. Only the new value of the ADU gets assessed. Your existing home keeps its

ADUs As Family Continuity Tools

SPEAKER_00 17:58

existing Prop 13 assessment. You are not resetting decades of tax protection by building in your backyard. County assessors published this guidance all over the place, so you can check it out online. Second, the reality that makes planning urgent, in November 2020, California voters passed Proposition 19, and it changed how property passes to your kids. According to the State Board of Equalization, the parent-child exclusion from reassessment now applies only if the child moves in and makes the home their principal residence. And even then it's capped. The current cap protects the home's assessed value plus about a million dollars. Anything above that gets added to the taxable value. Now run that against San Marino. If your home was assessed decades ago at a fraction of today's value, and today's market value is 3 million or more, that gap can blow right past the cap, which means the old assessment, the one that says kids will just inherit the house someday and it'll all work out, is shakier than it used to be. I'm not going to model the math on this episode. That's really not what it's about, because every family situation is different. And this is exactly the territory where you would need a professional at the table. But here's the strategic point I'm trying to make. And it's the whole reason I brought it up. The families who keep their ground in San Marino over the next 30 years would be the families who planned for it deliberately during their lifetimes, not the ones who assumed it would sort itself out. An ADU designed well and built right is one of the most concrete physical forms that planning can take. One more thing. And in San Marino, it's not a small thing. Quality and specifically getting it right the first time. Remember, what we covered earlier, this is a city whose building department has spent years looking for reasons to say no, and whose review standards by state mandate must now be objective. That changes what winning looks like. A design that meets every written standard on the first submission sails through a 60-day ministerial review. A design with soft spots invites exactly the friction this city is known for. Comment cycles, resubmittals, months of delay. In San Marino, design excellence isn't just aesthetics. It's the difference between building this year and building eventually. And the aesthetic bar should be high anyway, because this is San Marino. A cheap boxy unit dropped behind a 1920s estate home doesn't just fail the homeowner, it fails the street and the community. So I'd argue San Marino homeowners shouldn't be asking how to stop ADUs. That question has been settled. The harder and better question is if 287 of these are coming to my town, what does mine look like when it's done right? Designed to the standard of the neighborhood, engineered to pass an exacting review the first time, built to last as long as the main house and serving the family that already lives there. Built that way, an ADU isn't a threat to San Marino's character. It may just be its strongest defense. I want to close the loop on something personal. When my wife talks about growing up in San Marino, she's really talking about crossing that ocean. She was born in Taiwan and moved over with her family, and she doesn't lead with the architecture of the property or its property values. She talks about that feeling she gets when she's there, the quiet streets, walking to school, seeing the same families year after year. The sense as a kid that the world was stable, that adults had built something solid and you could just grow up inside of it. That feeling is the actual asset. Not the house, not the continuity. And here's what strikes me now. All these years later,

Prop 13 And Prop 19 Realities

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helping families plan spaces like this for a living, that feeling she carries wasn't an accident. It was built by parents and people who made deliberate choices about where to plant their families and how to keep them rooted. Some of those families have been on the same street since the Patents were neighbors. Others arrived from Taipei or Hong Kong, Shanghai, with living memory and made the same choice with even higher stakes. Different journeys, but identical instincts. Plant the family somewhere worth staying and hold on to it. At Actinidiu, we call this family integrity planning. And honestly, San Marino might be the purest example of why it matters. It's the idea that your property isn't just a financial asset. It's the physical foundation of your family's story. And with intention, it can keep serving that story across generations. Aging parents close but independent. Adult kids with a real path to stay. Grandkids growing up with the same sense of permanence their parents had. A town like San Marino was built by people who thought in generations. And it's home today to families who think exactly the same way. And to a city that takes pride in preserving that feeling. Honoring that legacy isn't about freezing everything in place. It's about making sure the next generation of your own family inherits more than a photo album. They inherit the ground. So with that, thank you. Thanks for joining us. If today's episode got you thinking whether your family has held its ground in a town like San Marino for 60 years or crossed an ocean to claim it, or you're just starting to imagine what continuity could look like on your property, I want you to know this kind of planning is exactly what we do. And ActonADU has been building for more than 35 years. And we don't just build ADUs, we build homes that last and legacies that outlast us. Family first, transparent, no surprises, one stop shop. If you're ready to explore what's possible, visit ActonADU.com or give us a call for a free consultation. And one quick reminder. Nothing in this episode is financial tax or legal advice for anything involving property taxes, Proposition 19, or your family's estate, please sit down with qualified professionals. Every situation is different, every property is different, and city regulations change. Always verify current local jurisdictions before you plan anything. Okay? So thank you for listening. Thank you for caring about

Design Quality Then Next Steps

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your family's future. And we'll see you next time. I'm James, and this has been ADU Adventures Building in the Backyard.