Trusted ADU Builder Serving Riverside County, CA
US Modular Inc. is a trusted ADU builder serving homeowners throughout Riverside County, California. Specializing in streamlined, cost-effective accessory dwelling unit solutions, they help property owners navigate the process of adding a granny flat, detached ADU, garage conversion, or casita to their existing property. With over 35 years of experience as a general contractor and real estate developer, US Modular Inc. brings deep expertise to every project.
The passage of Senate Bill 1069, which went into effect January 1, 2017, created new regulations that opened the door for homeowners to build accessory dwelling units in Riverside County. The legislation was prompted by escalating housing prices and a growing need for more affordable housing. Whether a homeowner is looking to house an elderly parent, create a rental income stream, or simply add usable living space, an ADU offers a practical and increasingly popular solution.
A typical client of US Modular Inc. builds an 850 sq. ft. accessory dwelling unit featuring a kitchen, living room, up to two bedrooms, and bathrooms — a manageable and affordable option for most homeowners in Riverside, CA.
Some key ADU facts for Riverside County include:
- ADUs cannot exceed 1,000 sq. ft. (standard ADU)
- An ADU can be a standalone unit or a garage conversion
- Reduced parking requirements apply, subject to several variables
- Reduced fees for water, sewer, and utility connections
- ADUs can be built using modular construction, site-built methods, or as a manufactured home
- Fire sprinklers may be eliminated if the main dwelling does not have them
- The ADU and main residence must still meet local zoning floor area and setback requirements
Riverside County also allows a Second Unit (SU) with a larger floor area than a standard ADU. Maximum floor area depends on lot size: up to 1,200 sq. ft. for lots between 7,201–20,000 sq. ft.; up to 1,500 sq. ft. for lots between 20,000 sq. ft. and 1.99 acres; up to 2,500 sq. ft. for lots between 2 and 3.99 acres; and up to 200% of the primary home's area for lots of 4 or more acres.
Feasibility Study and Full-Service ADU Construction Process
To help homeowners make an informed decision, US Modular Inc. offers a Feasibility Study with Conceptual Budget service. This Phase 1 service reviews local building department rules and regulations, analyzes floor area ratios, setbacks, parking, available utilities, and plan submittal requirements. It also includes creating a site plan of the current home, analyzing options for converting an existing garage into an ADU or pursuing new construction, and developing a conceptual budget for both scenarios.
If the feasibility study and budget are agreeable, US Modular Inc. moves into Phase 2 — full ADU construction. This phase includes:
- Choosing a pre-designed or custom ADU floor plan
- Determining the type of construction: site-built, modular, or manufactured
- Creating required civil engineering and house plans
- Submitting plans to state and local building departments for approval
- Demolition, grading, and utility installation
- Foundation construction and home build or modular/manufactured home installation
- Coordinating inspections with state and local building departments
- Completing exterior site work and landscaping per plans
- Obtaining a Certificate of Occupancy
Riverside ADUs start at $150,000. Site-built ADUs and garage conversions are priced based on design, finishes, and applicable code requirements.
What Is a Prefabricated ADU and Why Choose US Modular Inc.?
US Modular Inc. specializes in prefabricated ADU construction — building accessory dwelling units off-site in a controlled factory environment before delivering and assembling them on the homeowner's property. They build both Modular (Local Building Code) and Manufactured (Federal Building Code) ADUs. This method is faster, more cost-effective, and results in higher quality compared to traditional site-built construction.
A detached ADU — commonly referred to as a granny flat, guest house, second unit, or casita — is a separate structure on a single-family or multi-family property that already has a primary residential structure. US Modular Inc. also builds Junior ADUs (JADUs), which are constructed within the existing home. Their prefab approach allows site work such as grading, utility prep, and foundation work to proceed simultaneously with factory construction, significantly reducing overall project timelines.
The typical timeframe to complete a prefabricated ADU project with US Modular Inc. is 6 to 9 months. Their full-service process covers every step — from virtual site assessment and design finalization to permit submission, factory fabrication, on-site installation, and final inspection — ensuring a transparent and stress-free experience for every homeowner.