Custom Prefab ADU Company in California

Somewhere on your California property right now, there’s probably a garage that’s half-storage, a corner of the yard sitting idle, or unused space that could be earning rental income, housing a family member, or simply doing a lot more for you.

Building Prefab ADUs in California the Right Way

There’s no shortage of contractors who’ll promise you a fast, affordable ADU and then disappear into the permit process for six months.

Cornejos Builders has completed 24 ADU projects across Southern California over four years, and we’ve gotten good at making this feel a lot more manageable than it looks on paper.

We work with California homeowners from the very first conversation through final inspection. One team, one point of contact, one consistent process. You won’t be handed off to a subcontractor who’s never seen your property and isn’t sure what the last guy quoted you.

If you’re thinking about adding living space, generating rental income, or increasing the long-term value of your property, a custom prefab ADU from Cornejos Builders is a good place to start the conversation.

Alexsander Cornejo – Owner of Cornejo Builders

Hi, I’m Alexsander Cornejo, the owner and founder of Cornejo’s Builders. I started working in the remodeling and construction industry right out of high school, in 2009.

With Cornejo’s Builders, the goal is to give back to the city I grew up in. I want to send a message of hope and inspiration: any dream can be achieved, no matter where you come from, if you put in enough work and dedication.

Why a Prefab ADU Makes Sense in California

California’s housing market doesn’t give you much room to be slow about things. Between high construction costs, complex local regulations, and limited yard space in most neighborhoods, a prefab ADU is one of the more practical paths to adding real value to your property.

Here’s what actually makes modular ADUs worth considering:

Modular ADUs are manufactured in a factory while site work moves forward on your property at the same time. That overlap cuts weeks, sometimes months, off the total project schedule compared to a sequential stick-built process.

Construction happens in a controlled environment, which means less exposure to weather delays, labor scheduling issues, and material waste. You get more predictable pricing before the first shovel hits the ground.

Prefabricated units are built to tight tolerances. Better insulation, tighter seals, lower utility costs over the life of the structure.

Assembly of a modular unit on-site typically takes days, not months. Your primary home, your driveway, your yard, all see far less impact than a traditional construction project.

California’s updated ADU laws have made it considerably easier to add a unit to your residential property. If you’ve been on the fence about this for a while, the conditions right now are about as favorable as they’ve been.

Types of Prefab ADUs We Build

Every lot is a little different, and the right ADU type depends on your specific property, your city’s requirements, and what you actually need the space to do. Here’s a breakdown of what we build:

Detached ADU: A standalone structure placed in your yard, fully separate from your primary home.
Attached ADU: Built directly onto your main home and sharing one or more walls.
Garage Conversion ADU: Your existing garage becomes a fully code-compliant accessory dwelling unit, frequently without the need for a new foundation.
Junior ADU (JADU): Junior ADUs have their own entrance and basic kitchen facilities.
Above Garage ADU: A second-story unit constructed on top of an existing garage. These work well on narrow lots where yard space is limited.

See Our Work in California

Take a few minutes to browse through our completed projects. Seeing finished work is the fastest way to understand what Cornejos Builders actually delivers, and the range of design styles, sizes, and finishes we’ve pulled off across Southern California.

60+ projects done

100+ of families helped

1:1 approach to your project

Unsure about choosing us? Request a quote to get to know us better! Let’s discuss your project. Or, call us at (562) 319-3178 now!

What People in California Say About Us

Our clients will tell you more about working with us than any description we could write. Read through their experiences and get a real sense of what this process looks like from start to finish.

Esther Rosas

Rated 5 out of 5

Alex and his team were very easy to work with. They helped us build a JADU and ADU. He handled the planning and permitting of both units. As well as the inspections.

He was very helpful when we had questions and made sure to maintain excellent communication with us.

I would recommend Alex and his team to anyone looking for a good contractor. He’s also a pretty cool guy and will make you feel comfortable as these projects can make people uneasy.

Chris

Rated 5 out of 5

Alexsander did an amazing job for me, I hired him for my project in Downey. He was very professional, clean, responsive and got the job done in a timely manner.

As problems came up during construction, he presented solutions right away. He built from the ground a two-story apartment and converted a garage into an ADU.

The cost of the construction was decent and the quality in material and labor was great. I would hire him again and I have been looking for other projects to do with him already.

How Much Does a Prefab ADU Cost in California?

ADU construction in California typically runs between $80,000 and $250,000, depending on size, scope, and the design you choose.

Prefab ADUs tend to land at the more predictable end of that range compared to traditional builds, but there’s still real variation based on your property and goals.

Five factors that affect your ADU project cost:

Size and Square Footage: The floor plan shapes a significant portion of the budget. A compact junior ADU and a two-story 735 sq. ft. unit with vaulted ceilings are genuinely different projects from a cost standpoint.
Site Conditions: Your property's slope, soil type, existing utility connections, and setback distances all factor into site work costs. Some lots are more complex than others, and that shows up early in the estimate.
Finish Level and Custom Design: Standard finishes keep costs manageable. Custom design choices like smart home systems, marble countertops, or specialty tile can add up fast. We'll help you figure out what's worth spending on and what isn't.
Permits and Local Requirements: Permit fees vary across California. Los Angeles, San Diego, Beverly Hills, and Santa Monica each run their own schedules and processing timelines. We factor all of it in from the start so nothing surprises you mid-project.
Utility Connections: A new ADU requiring entirely new utility connections from the street costs more than one that ties into your existing setup. We assess this early so you know what to expect before anything is locked in.

Contact us with your property details, and we’ll put together an estimate that reflects your actual project, not a generic starting number.

Prefab vs. Stick-Built ADU: An Honest Look

Here’s a straightforward breakdown of how they stack up:

Factor	Prefab ADU	Stick-Built ADU
Build Time	Faster	Longer
Cost Predictability	More stable, factory conditions reduce weather and labor variables	More variable, with exposure to supply delays, weather, and crew scheduling
Customization	High, modern modular designs offer wide design flexibility	Very high, fully custom from the ground up
Site Disruption	Minimal, on-site assembly typically takes days	Significant, months of active construction on your property
Quality Control	Good, with factory inspections at each stage	Dependent on on-site conditions and crew consistency

For most California homeowners, the speed, cost stability, and reduced site disruption of a prefab ADU make it the stronger choice. That said, some projects are better served by a fully custom stick-built approach, and we’ll tell you honestly which one fits your situation.

Budget and Timeline: What You Can Actually Expect

The two things homeowners worry about most going into a construction project are costs running over and timelines stretching out indefinitely.

A prefab ADU is a real investment, and we’re not going to pretend otherwise. But when you think about $150,000 spread over 20 years of rental income or daily family use, it works out to less than $25 a day. We quote clearly, we don’t add costs mid-project, and we’ll always tell you what something costs before we move forward.

We’ve built 24 ADUs, and we know what realistic timelines look like. When something shifts, you hear from us the same day. Your property stays clean throughout the build, and the vast majority of our projects come in on schedule. Construction doesn’t have to feel like chaos, and with us, it usually doesn’t.

How the Prefab ADU Process Works at Cornejos Builders

Factory production, site prep, and permitting all run in parallel at Cornejos Builders. That means less time waiting and more time moving forward.

Collaborate

We review your property and your goals, then design a solution built around your specific site.

Build

Your modular ADU is manufactured off-site while permits and site work happen simultaneously.

Enjoy

Your unit is ready for a tenant, a family member, or whatever you had in mind.

FAQs About Prefab ADUs in California

How long does it take to build a prefab ADU in California?

Most prefab ADU projects run between 3 and 6 months from permit approval to final inspection. That timeline depends on your city, your lot, and design complexity. The key advantage of the modular approach is that factory production and site prep happen at the same time, which cuts meaningful time off the overall building process compared to a traditional sequential build.

Do I need permits for a prefab ADU in California?

Yes, permits are required regardless of whether your ADU is prefab or stick-built. California has streamlined the process in recent years, especially for standard designs, but every unit goes through plan check, permit filing, and inspection at each stage. We handle all of it.

Can a prefab ADU generate rental income on my California property?

Absolutely. California’s rental market is strong across most of the state, and a well-placed ADU can generate steady income that offsets construction costs over time. We can walk you through realistic numbers for your specific area before you commit to anything.

What is the difference between a modular ADU and a prefabricated ADU?\

The terms are used interchangeably most of the time. Technically, modular ADUs are built in sections in a factory, then transported and assembled on your property. Prefabricated ADU is the broader category covering any unit built off-site before delivery. In practice, both offer the same core advantages: factory-controlled quality, faster timelines, and more predictable costs than traditional on-site construction.

Build Your Prefab ADU with Cornejos Builders

Alexsander Cornejos

Cornejos Builders has been building custom prefab ADUs across California long enough to know how to get your build to the finish line without the stress that often follows construction.

Call us at (562) 319-3178 or visit our contact page to get the conversation started.

For a full picture of what we offer, take a look at our ADU contractor services.

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California is home to world-famous spots like Griffith Observatory in Los Angeles and Balboa Park in San Diego, as well as one of the most competitive real estate markets in the country. Owning property here means an ADU contractor in California is one of the best moves you can make.