ADU Construction and Home Additions by Cornejos Builders
Cornejos Builders is a construction company serving homeowners across Downey, Cypress, Cerritos, La Mirada, Whittier, and surrounding areas throughout Los Angeles and Orange County. With 24 ADUs completed in just four years, they offer a range of residential construction services including accessory dwelling unit construction, garage conversions, granny flats, bump-outs, second-story additions, porches, decks, and attached garages.
Their services are designed to provide homeowners with additional living space through a structured, permit-compliant process. Cornejos Builders handles everything from initial design to final construction, making it a turnkey solution for those looking to add an accessory dwelling unit or similar structure to their property.
What Cornejos Builders Offers
The company provides several types of ADU and home addition services, including:
- New detached ADU construction
- Garage conversion contractor services
- Granny flat and in-law suite builds
- Junior Accessory Dwelling Units (JADUs) up to 500 sq. ft.
- Second-story additions
- Bump-outs and attached garages
- Porches and decks
A standard new ADU built by Cornejos Builders typically includes a kitchen, bathroom, living room, washer and dryer hookup, one bedroom or studio layout depending on size, as well as central heating, water, gas, and electrical installations. The unit is delivered move-in ready on the day construction is completed.
For those looking for a more budget-conscious option, conversion ADUs — such as converting an existing garage or basement — and Junior ADUs (JADUs) restricted to a maximum of 500 square feet tend to be among the more cost-effective choices.
Cornejos Builders does not take over projects that were started by another contractor. If previous work needs to be removed before they can proceed correctly, they communicate that clearly upfront.
ADU Pricing and Project Examples
ADU construction costs with Cornejos Builders generally range from $80,000 to $250,000, depending on the size and scope of work. Below are a few examples of completed projects:
- 735 sq. ft. Two-Story ADU in Alhambra, CA — Quoted at $250,000. Included new foundation, plumbing, electrical, HVAC, Spanish-style finishes, custom kitchen cabinets, and separate utility connections. Architectural plans accounted for $15,000 of the total.
- 535 sq. ft. ADU in Whittier, CA — Quoted at $150,000. Featured central AC, open-concept kitchen and living room, custom bathroom tile, vinyl engineered flooring, and level 5 drywall finish. Custom kitchen cabinets with marble countertops were priced at $12,550.
- 798 sq. ft. ADU in Downey, CA — Quoted at $200,000. Included vaulted ceilings, smooth interior and exterior finishes, smart home electrical, curbless walk-in shower, and central AC. Bathroom tile was the most expensive single item at $8,000.
Because final costs depend heavily on design complexity, materials, and project scope, Cornejos Builders recommends requesting a free on-site consultation rather than relying on rough estimates.
Most ADUs can be completed in as little as 90 days, though timelines may extend depending on the complexity of the project and material choices. An exact timeframe is provided after an on-site visit.
Cornejos Builders is licensed, insured, and bonded. They manage all planning, permitting, and construction in-house, so homeowners do not need to coordinate separately with architects, permit offices, or subcontractors. All workers on-site are local tradespeople. A one-year warranty is included on all new ADU construction, and materials purchased and installed by the company retain their manufacturer's warranty.
3D design renderings are included in their design-build process, allowing homeowners to review and approve the layout and finishes before any construction begins.
