An ADU foundation is the structural base that carries an accessory dwelling unit, and getting it right is the difference between a permit that clears and a project that stalls. Benitez Concrete Construction is an ADU foundation contractor serving San Jose and the greater Bay Area, with more than 30 years of structural concrete experience. Whether your ADU is a detached new build, a garage conversion, or a unit tucked into a hillside lot, we pour the slab, grade beam, or pier foundation your plans and your engineer call for, built to California code and inspected to pass.
ADU Foundations Built for San Jose Lots
California has made it far easier to add an accessory dwelling unit, and the foundation is where most ADU projects meet their first real engineering decision. The right system depends on your soil, your setback, the size of the unit, and whether you are building new or converting existing space. A flat backyard with stable soil may take a standard slab. A sloped lot, expansive clay, or a unit sharing a wall with the main house often calls for a pier and grade beam foundation that reaches deeper, stable ground.
Benitez handles the full range of ADU foundation work: slab-on-grade, raised foundations, pier and grade beam systems, and the structural tie-ins required when an ADU connects to or sits near an existing structure. Every approach is part of the broader foundation construction that has been our specialty for three decades.
When You Need a Purpose-Built ADU Foundation
Not every ADU can sit on a simple slab. You likely need an engineered foundation designed for your specific lot if any of these apply:
- Your lot slopes, terraces, or sits on a hillside
- The soil is expansive clay or has a history of movement
- The ADU is a garage or basement conversion tied into the existing foundation
- The unit is two stories or carries significant structural load
- Your plans require the foundation to clear utilities, trees, or tight setbacks
If you are early in planning, our post on ADU foundation requirements in San Jose and the Bay Area walks through what the code expects and how the foundation choice shapes the rest of the build.
Why Bay Area Homeowners Choose Benitez for ADU Foundations
An ADU foundation is structural work, and the quality of the pour sets the ceiling for everything built on top of it. Benitez Concrete Construction has built residential foundations across the Bay Area for more than 30 years as a family-owned company. We hold California Contractor License #706549, classification C-8 (Concrete), and we are fully bonded and insured.
We work alongside homeowners, general contractors, structural engineers, and architects, which keeps ADU projects on time and on budget through permitting and inspection. Our portfolio includes ADU foundation work at the Hancock residence in San Jose and the Keenan residence in San Mateo, where the foundation had to serve both a new basement and an accessory unit. You can read more about our team and how we approach structural concrete.
Our ADU Foundation Process
Every project is different, but the path from plan to poured foundation follows a clear sequence:
- Plan and engineering review. We review your ADU plans and soils report with your engineer and confirm the foundation design that fits the lot.
- Permitting support. We work within the city’s ADU requirements so the foundation meets code and inspection at each stage.
- Site prep and excavation. We excavate and grade the pad, and shore where needed to protect nearby structures.
- Forming, rebar, and pour. We form the foundation, set reinforcement to spec, and place the concrete to the engineered design.
- Inspection and handoff. We pass the required inspections and hand your builder a foundation ready for framing.
ADU Foundations Across the Bay Area
Benitez builds ADU foundations in San Jose, Atherton, Cupertino, Hillsborough, Los Altos Hills, Los Gatos, Menlo Park, Mountain View, Palo Alto, and Redwood City. Because so many local ADUs are added to established homes on challenging lots, our foundation work often overlaps with residential concrete construction and, where a homeowner is adding living space below grade, with our basement construction.
Frequently Asked Questions
What kind of foundation does an ADU need?
An ADU typically sits on a slab-on-grade, a raised foundation, or a pier and grade beam system, depending on the soil, the slope, and the size of the unit. Flat, stable lots often take a slab, while sloped or expansive-soil lots usually need an engineered pier and grade beam foundation. A soils report and structural plans determine the right choice.
Can you build an ADU foundation on a sloped or hillside lot?
Yes. Sloped and hillside lots are common across the Bay Area, and they usually call for a pier and grade beam foundation that reaches deeper, stable soil. We coordinate with your structural engineer to design and pour a foundation that handles the grade, the setbacks, and the load safely.
Do you handle garage-to-ADU conversions?
Yes. Garage conversions often need the existing slab evaluated and reinforced, or a new foundation poured where the original does not meet the code required for living space. We assess the existing foundation, work with your engineer, and build the structural base the conversion needs.
How long does an ADU foundation take?
Every project is different. The timeline depends on the foundation type, the soil, site access, and permitting. After we review your plans and the lot, we give you a realistic schedule. A site visit is the only way to estimate your project accurately.
Do you work with our architect and general contractor?
Yes. We regularly partner with architects, structural engineers, and general contractors, and we can slot into your existing team. Our role is the concrete and structural execution, built to the engineer’s specifications and inspected to code.
Start Your ADU on Solid Ground
A well-built foundation keeps your ADU project on schedule and out of trouble with inspectors. Contact Benitez Concrete Construction for a free estimate, or call (408) 271-9826 to talk through your ADU foundation with a team that has poured Bay Area foundations for over 30 years.