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 dealing with 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 prefab ADUs becoming more popular in Southern California?

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 prefab ADUs allow for 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, are frequently turning 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’s how some of the benefits of building prefab ADUs:

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 Prefab ADU Saves Time

’Prefab’ is shorthand for ‘Previously Fabricated’, meaning it was built in a factory and is reassembled or finished onsite. Our ADUs are built to completion in our partner factories, transported to your site, and can be installed to a foundation within 24 hours. Prefab construction has the advantage of 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 disruptive to residents of 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 of 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 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 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 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

Site specific conditions, local jurisdiction requirements, lender requirements and décor/finish/options selections may require additional costs.

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, 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 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.

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 get asked daily. For us it’s a pretty easy answer, because we can build better, faster, greener and for less in a factorythan 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 then when they’re assembled in your backyard. Another important reason to consider a prefabricated ADU 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 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 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 – click link:


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

Why choose USModular Inc to build your 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.

Call us at 951-679-9907 x 1 to discuss your project or fill out our questionnaire to schedule a site visit. We will talk about budget, intended use of ADU, ADU location on property and access to your site.

Click here to fill out our quick start questionnaire!

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