Pre-Approved ADU Plans Make It Easier Than Ever to Build in California

Pre-Approved ADU Plans Make It Easier Than Ever to Build in California

If you’ve ever dreamed of adding a guest house, in-law unit, or rental cottage to your property, there’s good news. Building an Accessory Dwelling Unit (ADU) in California is more accessible than ever.

Thanks to a new state law (AB 1332) that took effect this year, homeowners now have the option to choose from a set of pre-approved ADU plans provided by local jurisdictions. That means less red tape, faster approvals, and significant savings upfront.

What’s Changing?

Traditionally, homeowners could spend thousands of dollars and wait months just to get architectural plans drafted and approved. Now, under this new legislation, cities and counties must offer a library of pre-approved plans, and once you select one, your permit must be approved within 30 days.

For comparison, custom ADU plans often take 60–90 days for review and can cost $5,000 to $15,000. Pre-approved plans typically range from $1,000 to $7,000 and in some cases, like those offered by the nonprofit HEART of San Mateo County, they’re even free.

Why This Matters

ADUs have become a go-to solution for families looking to create multigenerational living spaces or homeowners interested in generating rental income. In fact, ADUs made up 1 in 5 new housing units built in California last year.

But even with the streamlined process, building an ADU is still a major project. Construction can range from $500 to $600 per square foot. Meaning a small 600 sq. ft. unit might cost $300,000, plus utility hookups and site prep. Still, for many, it’s a more attainable and flexible option compared to the high cost of traditional housing in the Bay Area.

Local Resources to Explore

Here in the South Bay, 10 cities have teamed up to create Santa Clara County ADU. It is an online resource that showcases pre-approved backyard units of all sizes. From compact studios to two-bedroom layouts, it’s a great place to explore design options and get inspired.

Cities like Oakland, San Mateo County, and Mountain View also share links to architectural plans directly on their websites. These resources are designed to remove the guesswork for homeowners and make the path to building an ADU far less intimidating.

What to Know Before You Build

Even with a pre-approved plan, you’ll still need a site plan showing where your ADU will be placed and how it connects to water, sewer, and electrical systems. Some designers or prefab ADU providers include this service, while others may charge separately.

And while estimated costs attached to these plans might look appealing, it’s important to dig deeper. California’s diverse soil types and seismic zones can add complexity to your project. Asking the right questions up front can help you avoid surprises down the road.

Bottom Line: Pre-approved ADU plans can help homeowners save time, reduce costs, and move from idea to reality much faster. Whether you’re planning to create space for family or generate extra income, the tools and support are now more accessible than ever.

Thinking about building an ADU? We’d be happy to help you explore your options and connect you with trusted local resources. Let’s talk! Reach us at (408) 297-5638 or [email protected]. You can else send us a message on our Contact Us page.

Sources: SCCADU, HEART of SMC, Mecury News