Full-Service Prefab ADU Builders in Southern California

Welcome to USModular Inc, Southern California’s premier full-service design and build general contractor specializing in the construction of single-family manufactured homes and prefab Accessory Dwelling Units (ADU). We have built hundreds of prefab ADUs in Southern California using prefabricated construction and are experienced and knowledgeable in all aspects of the building process.


What is a Prefab ADU?

A prefabricated Accessory Dwelling Unit — commonly known as a Granny Flat or Companion Unit — is a second small dwelling on the same parcel of land as your single-family house. An ADU can be up to 1,200 sq ft and can be built as:

  • Detached stand-alone building
  • Attached to the primary dwelling
  • Converted space within the primary dwelling (Junior ADU)

Washington – Colonial Series Flyer – Prefab ADUs in Southern California


Why are Modular ADUs in California Becoming More Popular?

Thousands of new prefab ADUs are being built in communities across Southern California because they are a manageable and minimally disruptive option for adding needed housing to existing neighborhoods. Moreover, ADUs typically cost less than other forms of housing and make efficient use of existing land. The other factor driving ADU adoption is that they serve so many varying needs that mirror socioeconomic trends in the U.S. Modern, modular ADUs in California allow people to age in place or form multi-generational households to ease caregiving responsibilities and cost. Homeowners, given the high cost of buying a house, frequently turn to ADUs to provide additional income to make ends meet. Lastly, the appeal of downsizing and minimalist living seems to be growing, and ADUs allow people to simplify their lives and still live in a neighborhood setting.

Here are some of the benefits of building prefab ADUs in California:

Additional Space For Family Members – Grandparents, Kids, And Relatives

We recently built an ADU for a family in Orange County. It was built for the homeowner’s Mom, who was living in a 1 bedroom apartment – 10 miles from family and they were spending $1,500 a month on rent.  Grandma moved into The Bungalow ADU and is now closer to her daughter and grandkids. She feels safer, and has more social connections with family. The family has a net savings of over $1,000 a month in rent. A true ADU success story.

Rental Income – turn a single-family rental property into a 2-income property.

An Investor with a single-family income property in San Diego put it best when he asked us – “where can I buy a 2-bedroom rental property for under $120,000 in San Diego? Nowhere! But I can build a 2 bedroom ADU for this price – it’s a no brainer”

The presence of an ADU can often enhance the attractiveness of a house on the market, and increasingly, investments in new ADUs are being reflected in the appraised value of those properties.

LAKESIDE – Prefab ADU Investment Property Flyer

Building a Manufactured ADU in Southern California Saves Time

’Prefab’ is shorthand for ‘Previously Fabricated’, meaning it was built in a factory and is reassembled or finished onsite. Our manufactured ADUs in California are built to completion in our partner factories, transported to your site, and can be installed on a foundation within 24 hours. Prefab construction has the advantage of being significantly faster and more efficient than other construction types. Building in a factory allows us to use efficient processes and avoid weather delays so that we can complete your project on time and on schedule!

Cost Savings, Time Savings and less disruption to residents of the Primary House and neighbors are big advantages of using Prefab construction.

New 2023 California Laws:

ADUs are less expensive, faster and easier to build!!!

California is in a housing crisis, both in physical houses and affordability. ADUs help the crisis by increasing the number of homes available and lowering costs to build as land and utility infrastructure is already in place (2 of the biggest costs to build a new home).

New State of California ADU laws include:

  • ADUs can be up to 1,200 sq. ft. regardless of the size of the primary dwelling unit (as long as you meet setbacks and FAR)
  • Setbacks reduced to 4’ side and rear yard
  • Parking requirement for ADU waived
  • No owner occupancy of primary dwelling unit (great for investors)
  • 60-day review and approval process by the local building department, or considered approved
  • Waiver of local development fees for ADUs less than 750 sq. ft.
  • Multi-family homes eligible for up to two ADUs
  • Fire sprinklers are ONLY required if the primary dwelling unit has them and your ADU is less than 150′ from the curb.

Costs of building Prefab Accessory Dwelling Units

USModular Inc. offers prefabricated detached Accessory Dwelling Units starting at $155,000 for an ADU built to the HUD/Federal Code with Delivery, Foundation System, Installation, and Utility Connections included for projects within our Southern California service area.

  • Delivery
  • Foundation System
  • Installation and Utility Connections

Additional costs may be required for site-specific conditions, local jurisdiction requirements, lender requirements and décor/finish/options selections.

USModular Inc offers a no obligation project site review for ADU projects within our Southern California service area.

Is your project located within the city limits of San Diego, Los Angeles or Riverside?

Tiny homes are approved as ADUs in these areas! We offer Tiny Homes aka Park Model ADUs
starting at $145,000 with Delivery, Installation and Utility Connections included (CA DMV fees, title, tax, and registration are additional costs). Site-specific conditions, local jurisdiction requirements, lender requirements and décor/finish/options selections may require additional costs.

All pricing is subject to change without notice.



Frequently Asked Questions: Prefab ADU


Are ADUs allowed in my southern California neighborhood?

In 2019, California passed a series of bills to make it easier for homeowners to build ADUs. Major changes to zoning regulations, permit requirements and developmental standards went into effect on January 1st, 2020, superseding previously restrictive local regulations. While zoning, regulations and fees vary by municipality, the recent state laws say that ADUs can be built on property zoned for single-family and multi-family use.

Learn about Southern California ADU regulations & rules for your city:


Can I build an ADU? How many can I build?

If you live on a residential lot, yes! You can build one detached ADU and one junior ADU (a unit no more than 500 sq ft located within the home). If you live on a multi-family lot, a maximum of two detached ADUs are allowed on the property.


Why build an ADU with prefabricated construction?

This is a question we ADU builders in southern California get asked daily. For us, it’s a pretty easy answer because we can build better, faster, greener and for less in a factory than a site-built ADU. Same reason that we drive cars built in a factory: they have a more precise process building on a production line, indoors, by a team of workers that work in sync than when they’re assembled in your backyard. Another important reason to consider an adu prefab in California is that the site construction is minimal (foundation and utilities), while a site-built project will take months of daily hammering, sawing, dumpsters, and port-o-potties with 10+ different workers walking in and out of your backyard. With prefabricated construction, the ADU is built in a factory, and we will set the unit in your backyard in one day. Once the ADU is set we will have a very small crew do finish the work and complete utility connections.


How long will it take to build an ADU?

A lot depends on you. How long you take to choose the style, building code (ANSI, Local, and Federal), and floor plan will determine the length of the project. Once the ADU plan is chosen and a contract is signed, it typically takes from 6-9 months to get a building permit and start the project. Much of this time is spent on the State and Local approval of the building plans. Once we have the permit in hand, the USModular team will act very fast and typically can complete a project within 3-6 months.

We have 100’s of predesigned Floor plans for ADUs.

See our ADU Floor plans at www.myadu4less.com!


What is the process to build an ADU with USModular?

Phase 1: Planning

  • Site visit to determine setbacks, accessibility, utility location /availability, FAR
  • ADU submittal requirements
  • New construction of ADU with Modular (CBC/Local) or Manufactured (HUD/Federal))
  • Develop a conceptual budget for ADU and site construction

Phase 2: Construction Contract

  • Finalize construction budget and contract

Phase 3: Plan Design and Submittal

  • Site plan design
  • ADU approval through State approved 3rdparty and local jurisdiction

Phase 4: Construction

  • Build (Fabrication) the ADU at Factory while prepping onsite construction necessary to meet local ADU requirements, including, utilities, foundation
  • Coordinate with the local building department for inspections

Phase 5: Final Inspection

  • Walk through ADU with the client and make adjustments as needed
  • Obtain a Certificate of Occupancy


How do I start the process?

We thought you would never ask!

Review the series of floor plans that we offer:

  1. Review the series of floor plans and All-IN pricing that we offer at www.myadu4less.com
  2. Schedule a site visit to further review the project.


Why choose USModular Inc. to build your Modern Prefab ADU in Southern California?

With 75 combined years in general contracting and real estate development, we learned a few things along the way. Building faster will save money, and using prefabricated construction allows the USModular team to work more efficiently, which translates to cost savings. At USModular Inc., we handle all aspects of your project. Starting with an initial site visit, we handle:

  • Budget creation
  • Contracts
  • Plans and permitting
  • Factory fabrication process
  • Site construction
  • Utility connections
  • And finally, handing you a set of keys to your new prefab ADU

Our units are built with minimal disruption to residents of the primary dwelling unit as the ADU is set into your backyard in ONE DAY!!!!! We are truly a one-stop shop.

USModular Inc. is a premier contractor of prefabricated homes and ADUs with over 40 years of experience in the industry. You can see the available floor plans and pricing at MyADU4Less.com. Contact us today to discuss your project, or fill out our questionnaire to schedule a site visit!  


Toll-Free: (888) 987-6638

Local: (951) 679-9907 Ext. 1


Check out our latest ADU projects!


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