What an ADU foundation actually involves

Foundations look simple from the outside. They're the most complex and consequential phase of any ADU build. Everything above the slab (framing, mechanicals, finishes, the entire cost of your ADU) depends on the foundation being poured correctly the first time.

A typical detached ADU foundation scope includes:

• Site prep and excavation. Clearing, rough grading, and digging the foundation footprint to the depth specified in your engineering.

• Utility trenching. Sewer, water, electrical, and sometimes gas lines run from the main house to the ADU footprint before the slab goes in.

• Formwork. Wood or metal forms set to match the plan dimensions, with correct elevations and slopes dialed in.

• Rebar. Reinforcement steel tied according to the structural engineer's specifications (size, spacing, and tie pattern), all inspected before any concrete goes down.

• Pre-pour inspections. Most jurisdictions require an inspection of the forms, rebar, and utility rough-ins before you can pour.

• Concrete pour and finishing. Concrete delivered, placed, consolidated, screeded, floated, and finished to the specified texture. Timing and weather both matter; a botched pour can't be undone.

• Post-pour curing and protection. Concrete gains strength over days and weeks. How it's protected during curing affects the final durability of the slab.

Getting any of this wrong creates problems that are either expensive or impossible to fix later. That's why foundation work is the phase where subcontractor quality varies most, and where a lot of ADU projects run into their first real issues.

Why in-house concrete matters

Most ADU builders subcontract their foundation work to whichever concrete crew is available that week. That's the standard industry model, and it creates three predictable problems:

Quality variance
Every subcontractor crew is different. Some are excellent, some are careless, and the builder can't always tell in advance which one showed up that morning. When the builder has no direct control over the crew, quality control is reactive instead of preventive.

Scheduling dependencies
If your builder's concrete sub is tied up on another project, your slab pour slips. If the sub is short-staffed, your pour slips. Weather, truck availability, and a hundred other factors outside your builder's control can push the schedule.

Markup on someone else's work
When a builder hires a concrete sub, they mark up the sub's price to cover coordination and risk. That markup goes to the builder, not to better concrete work on your project.

TrueBuild's model removes all three. Supreme Concrete Group is our sister company: same third-generation concrete professional owner, same lean approach, same quality control. We don't hire out our foundations. We pour them.

That means:

• One team from excavation to pour: no coordination gap between "site prep crew" and "concrete crew"

• Equipment and trucks we own and schedule ourselves: not booked through a sub's calendar

• Quality we can guarantee: because we're the ones doing the work

• No subcontractor markup: you pay for the concrete work, not for a builder's layer on top of someone else's labor

• A crew that's done this work hundreds of times before TrueBuild existed: Supreme Concrete Group has an established track record in San Diego County

This isn't a marketing angle. It's the single biggest structural difference between TrueBuild and every other ADU builder in the San Diego market.

What we can pour for your ADU

Slab foundations
The most common ADU foundation type in San Diego County. A reinforced concrete slab sits directly on prepared soil, with thickened edges and footings engineered to the structural specs. Works for most flat and moderately sloped lots.

Monolithic slabs
A variation where the slab and footings are poured as a single continuous pour. Common for ADUs because it's faster, more efficient, and reduces cold joints in the concrete.

Raised foundations
Sometimes required on expansive soils, hillside lots, or sites where the ADU needs to sit above existing grade. Stem walls with crawl space, or full piers-and-grade-beams, depending on the engineering.

Retaining walls and grade work
When a site needs to be leveled or terraced before the ADU can sit on it, we handle the concrete retaining walls and grade prep as part of the same project. No separate contractor, no separate bill.

Site prep and demolition
Clearing, removal of existing concrete (driveways, patios, old slabs), rough grading, and utility trenching. All of it feeds into the foundation phase and all of it is handled by the same team.

Foundation-only ADU projects

If you're running your ADU project as an owner-builder or working with a different general contractor, we can handle the foundation phase on its own. You coordinate the rest of the build; we pour the slab, sign off on the pre-pour inspections, and hand off a ready-to-frame foundation. For the full menu of what we offer, see all our ADU services.

Who this service fits best

This service is the right fit if:

• You want the foundation phase of your ADU done by people who pour concrete for a living, not a general contractor subcontracting it out.

• You're hiring us for a full detached ADU build or a shell build, in which case the foundation is included as part of the build scope.

• You're running your ADU as an owner-builder and need a trusted pro for the foundation phase specifically.

• You're working with another general contractor who will let you bring in a separate foundation contractor. (This is less common but possible on some projects.)

• You have a complicated site (hillside, expansive soil, tight access, retaining walls required) where foundation quality and scope matter even more than usual.

When someone else might be a better fit

If you're looking for a concrete contractor for a driveway, patio, or non-ADU residential concrete project, Supreme Concrete Group handles that work directly rather than through TrueBuild ADU. Our foundation services here are specifically scoped for ADU construction.

What sets TrueBuild apart on ADU foundations

Same owner on both companies
Jose Flores owns TrueBuild ADU and Supreme Concrete Group. There's no joint-venture agreement, no vendor relationship, no separate business interests pulling in different directions. It's one operation with two brands covering two scopes of work.

A concrete crew that existed before the ADU builder did
Supreme Concrete Group has been pouring concrete in San Diego County for years. TrueBuild ADU is the newer brand built on top of that established concrete expertise. The foundation capability isn't something we tacked on to an ADU business, it's the foundation that the ADU business was built on.

Equipment and vehicles we own and operate ourselves
We don't rent our vehicles and equipment by the day, or share it with other companies. Our excavators, skid steers, dump trucks, work trucks, and tools are owned, maintained, and scheduled by us alone.

Pre-pour reviews you can trust
Before we pour, we walk the job with you (or your general contractor if you're hiring us foundation-only) and confirm what's about to go in the ground. We'd rather catch a problem at form inspection than after the concrete has cured.

Jose on-site for every pour
Concrete pours are the moments where ADU foundations either go right or go wrong. Jose is on site for the pours on his own projects, and his presence is what Supreme Concrete Group's reputation was built on. That doesn't change when the work is scoped under TrueBuild ADU.

What to expect on timeline

Foundations on detached ADU builds typically take 2–4 weeks from site prep to finished, cured slab ready for framing:

• Site prep and excavation: 3–7 days, depending on site complexity, access, and how much demo or grading is needed.

• Formwork and rebar: 3–5 days. Forms set, rebar tied, utilities trenched and roughed in.

• Pre-pour inspections: 1–3 days, depending on inspector availability in your jurisdiction.

• Pour and finish: 1 day for a typical ADU slab. Concrete is placed, finished, and protected in a single work day.

• Cure: 5–7 days minimum before framing can start. Concrete keeps gaining strength for weeks after that, but framing can begin once it hits structural strength.

Things that shift foundation timelines: extensive demo, hillside sites, expansive soil requiring special engineering, weather/rain, utility trenching complexity, and permit/inspection backlog.

What affects ADU foundation cost

ADU foundation pricing depends on:

• Size and complexity of the slab or foundation system called for in your engineering.

• Site prep required: clearing, demo, grading, and utility trenching all feed into the foundation phase.

• Soil conditions: expansive soil, fill requiring compaction, or hillside conditions all add cost.

• Access to the site: tight driveways, narrow gates, and long concrete pump runs cost more.

• Engineering specifications: deeper footings, heavier rebar, or specialty structural features (shear walls starting at the slab, for example) all cost more than a standard slab.

When you hire TrueBuild for a full build, the foundation is priced inside the overall build scope. When you hire us foundation-only, we bid the foundation scope specifically based on your engineering and site.

What you won't pay: a subcontractor markup. Because we pour the concrete ourselves, you pay for concrete work directly, not for a layer of builder markup on top of a sub's price.

Common questions about ADU foundation work

Can I hire you just for the foundation phase if I'm using a different builder for the rest?
Yes, with some caveats. Your general contractor needs to be OK with a separate foundation contractor, your project needs to accommodate the scheduling, and the contract scopes need to be defined cleanly (who owns the pre-pour inspection, who signs off on the rebar, etc.). If all that works out, foundation-only projects are something we regularly take on.

Do you do raised foundations, stem walls, and pier-and-grade-beam systems?
Yes. Most ADUs in San Diego County end up on slab foundations, but raised foundations come up on hillside lots, expansive soils, and cases where the ADU needs to sit above existing grade. We handle all of these, and our engineering partners can help spec the right foundation type for your site if you don't already have engineering done.

What if my soil is expansive or needs special engineering?
Expansive soil is common in parts of San Diego and Riverside County. The solution is usually in the engineering: deeper footings, more rebar, sometimes a post-tensioned slab, sometimes a raised foundation instead of a slab. The engineering comes from a licensed structural engineer, not from us. We build to whatever the engineer specs.

Can you handle retaining walls as part of the foundation scope?
Yes. Retaining walls for hillside ADUs, terraced sites, or grade transitions are part of the same concrete scope on many of our projects. Handling them on the same project means the concrete crew, engineering, and timing are all coordinated rather than split across multiple contractors.

How is Supreme Concrete Group related to TrueBuild ADU?
Same owner (Jose Flores), different scopes of work. Supreme Concrete Group pours concrete for residential and commercial projects across San Diego County, and has been doing so for years. TrueBuild ADU is the dedicated ADU-building brand that draws on Supreme Concrete Group's crew for every foundation. To be clear, it's not a preferred vendor or partner, it's one operation run by one owner with two brands covering two scopes.

What happens if the city inspector flags something on a pre-pour inspection?
We fix it before the pour. That's the whole point of the pre-pour inspection, and catching issues at that stage is the cheapest and easiest time to catch them. If something's off in the rebar placement or formwork, we correct it and re-inspect before we schedule the concrete.