ADU Contractor

Get a home addition, a granny flat, a garage conversion, or an ADU construction in Downey, San Marino, Rancho Palos Verdes, Newport Beach, and more!
Home addition with vinyl floor and white kitchen cabinets

Scroll down to discover options, pricing, and process

Integrated living room and bedroom featuring sleek white closets that optimize storage space. The wooden flooring adds warmth and complements the modern aesthetic, while the black countertops provide a sophisticated contrast. This harmonious blend of elements results in a contemporary and functional living space, where efficiency meets stylish design.

Give Everyone You Love The Extra Living Space They Need

When we reach a certain age, our needs change as well. We’ll require more living space, a larger home, or an extra bedroom.

And if you want an ADU, one of the most important things to look for is quality.

Cornejo’s Builders don’t just deliver the highest-quality craftsmanship here in South California, but also a smooth and seamless construction experience you’ll love.

Here are some ADUs you can get:

Quality ADUs From A Trusted Pro

Frequently Asked Questions

Generally, we can build most ADUs in as quickly as 90 days. However, it’s important to understand that depending on the scope of work, complexity of the project, or your choice of materials and features, the timeline will increase.

To avoid any guesswork, we’ll give you an exact time frame after our on-site visit.

Everyone who will step on your property are trusted, local tradesmen. We only work with the best in the industry, all experts who have your best interests at heart.

Yes. However, it’s important to know that if you buy the materials yourself, this automatically voids the warranty. 

If we’ll get the materials and install them for you, you’ll still enjoy their warranty, which can last anywhere from 1 year to a lifetime.

Of course! You’ll be able to see a 3D design of your new ADU so you don’t have to commit to any materials or design until you’re 100% satisfied with your upgrade. 

The 3D renderings are included in our design-build process to ease your experience.

Yes! You’ll get a 1-year warranty on all new ADUs built by us, and the materials warranty on all materials that we’ll buy and use. 

On top of that, you’ll always have our support, so whenever you need us again, you’ll have priority over anyone else.

A new ADU here in South California includes a kitchen, a bathroom, a living room, washer and dryer hook up, 1 bedroom or studio (depending on size of ADU), central heating, water, gas, and electrical installations.

We’ll take care of everything, and you’ll get a turnkey ADU that you can use the very same day we’re done working!

ADUs vary from $80,000 up to $250,000.00 depending on scope of work and size. 

Ballparking the final cost is never a good idea, so it’s best to just request a free quote, have a consultation, and find out a realistic price for your upgrade.

Yes, all ADUs will require a permit here in South California. No need to worry though. Cornejo’s Builders Inc. handles all planning, permitting, and construction, so you don’t have to worry about anything. 

Just sit back, relax, and watch your dream home come to life.

Top benefits of building an ADU:

ADU Pricing Insights Here In South CA

While ballparking an estimate never works out, we still want to show you what some of our ADU projects include and what they look like. Check below!

New 735 sq. ft. Two-Story ADU in Alhambra, CA

This project included:
This project was quoted at $250,000, and the most expensive part of the project was the architectural plans we designed at $15,000.

New 535 sq. ft. ADU in Whittier, CA

This project included:

This turnkey ADU was quoted at $150,000, where the custom kitchen cabinets with marble countertops were priced at $12,550.

New 798 sq. ft. ADU in Downey, CA

This project included:

This project was quoted at $200,000, where the most expensive item was the bathroom tile which came out to be $8,000.   

Fully Enjoy The Excitement Of Your Home Upgrade

Let’s take you on a beautiful experience where all you need to do is enjoy your new ADU. This 3-step process ensures a smooth and stress-free experience, start to finish.



Contact us and let’s plan your ADU together with our designers.



Remain comfortable and on top of everything at all times.



Have everyone over for dinner and enjoy your new ADU!

A Lifelong Partnership With A Reliable Expert

Guarantee your project’s success with an expert who puts you and your investment first. Our continuous education ensures we deliver on our promise every single time.

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Lanny Lam​


Cornejo’s Builders did an amazing job remodeling our kitchen. Alexsander made this experience one of a kind, he was accessible and flexible throughout the process. 

This guy’s knowledge truly guided us to perfect our dream kitchen. He has endless recommendations. The workers were efficient. We appreciate how he handled minor changes we had with professionalism. 

After getting work done by different General Contractors, Cornejo’s Builders definitely sets the bar for the rest. Thank you, Alexsander.


Lety Ortega​​​


Called Cornejo’s Builders after seeing their beautiful work on Tik Tok. Spoke to the owner regarding an ADU project. Alex always returned my calls, explained throughly how the project would be completed. 

Love how his company is an all in one stop. They definitely handle everything from getting permits, architect, plumbing, electrical etc. Turn key ready, his prices are very reasonable and is willing to work around your budget

Alex was very understanding when I explained why I had to put a hold on the the project. He definitely treats his customers like family.


Yahritza Orellana​


Had a truly great experience with Alex. Cornejo’s Builders just completed converting my crawlspace/basement into an ADU (studio with new bathroom and kitchen).

What made the experience even more valuable was that the owner himself (Alex) came over for a consultation. Other companies just send their sales reps and most of the time they don’t even know anything. Feeling important is always a positive I can’t wait to work with them again. 

I highly recommend them. You can tell Alex is passionate about what he does, his clients, and his company.

The Perks Of A Seasoned ADU Contractor In South CA

Take advantage of the best perks of working with a local, trusted ADU professional here in Downey, San Marino, and surrounding areas.

Everyone involved in your project is 100% committed to your satisfaction. You’ll be treated with utmost respect 24/7.

You’ll enjoy the best tradesmen here in South California, all ready to deliver the best product you could ever find around here.

Remain comfortable while fully enjoying the excitement of getting an ADU. We have the know-how to make it happen!

Get The Extra Square Footage You Want Without Stressing Over Anything

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