A lot of families in Chula Vista are talking about a granny flat right now. Maybe it’s for a parent who wants to live close, a grown kid saving up, or a little rental income to help with the mortgage. The talk usually starts with the fun part: one bedroom or two, where the kitchen goes, whether to convert the garage or build out back.
Here’s a gentle nudge before you fall in love with a floor plan. Ask what the unit will sit on. On the east side especially, the ground can swell and shrink with water, and a small building is just as sensitive to that as a big one. Sometimes more so.
The rules changed shape in 2025
If you looked into ADUs a few years ago, you may remember a long section of the City code full of local standards. That’s gone. In April 2025 the City Council adopted Ordinance 3592, which repealed the City’s own ADU and junior ADU sections. The City’s legislative history lists it simply as a repealer of sections 19.58.022 and 19.58.023.
What that means for you, in plain terms: with the local sections gone, zoning questions like size, setbacks, and how many units a lot can hold now come from state law, which the state housing department summarizes in its ADU Handbook. The handbook is a good, readable place to start.
What it does not change is the building code. The unit still has to be designed and built to the California codes, and that includes the foundation. State ADU law makes it easier to get permission to build. It doesn’t make clay behave.
What the clay does
The California Geological Survey describes the Otay Formation, which underlies parts of eastern Chula Vista, as containing beds of light-gray bentonite claystone. Bentonite takes on water and swells, then shrinks as it dries. Soils derived from it can push up on a slab in the rainy months and drop back in late summer.
The County’s 2023 hazard plan annex for the city lists expansive soils as extensive in area but weak in magnitude, and ranks it low overall as a disaster hazard. That’s a fair way to think about it. Clay isn’t going to knock a house down. It is a slow, patient source of cracks, sticking doors, and uneven floors, and a new foundation should be designed with it in mind from day one. Our expansive soil foundation repair page explains the movement in more detail.
Why the soil report comes first
The residential code allows the building official to ask for soil tests where expansive soil is likely, and it sends slabs and footings on expansive soil to a separate engineered design provision rather than the simple prescriptive tables. The City’s own code lets it require a soil investigation with land development plans, and its grading rules say expansive soil found close to finished grade in a building area generally has to be removed and replaced with compacted non-expansive soil, unless a soils engineer’s alternative design is accepted.
So for a detached ADU on a clay lot, expect the soils question to come up. Even where nobody requires it, a short soils report can save money later by telling the designer which foundation type fits the lot instead of guessing.
If your house came with a geotechnical report from when the tract was graded, dig it out. Many east-side neighborhoods were mass-graded in recent decades, and those reports often describe the soil under each pad, including any fill. They don’t replace a fresh look at the ADU spot, but they’re a head start.
The garage conversion catch
Converting the garage feels like the easy path because the walls, roof, and slab are already there. Look closely at that slab, though.
The residential code generally calls for a vapor retarder under concrete floors, but it lists garages among the exceptions. So the slab under many garages was poured with nothing between the concrete and the soil. It was meant for cars, not bedrooms. That doesn’t rule out a conversion. It does mean moisture coming up through the floor, the condition of any cracks, and whether the slab has moved are all fair questions before you lay flooring over it. Our post on garage slab cracks walks through how to read that concrete.
If the house sits on a post-tension slab, which is common in Eastlake, Otay Ranch, and other newer east-side tracts, the garage slab may be part of the same tensioned system. Cutting a trench for a new bathroom drain is not a casual job there. Read our post on why you shouldn’t cut a post-tension slab before anyone picks up a saw.
Where the new meets the old
Attached ADUs and additions have one more wrinkle: the joint between the new foundation and the existing one.
A brand-new footing on freshly prepared soil and a decades-old slab on settled ground will not move in perfect step. On clay, the difference can show up as a crack along the joint, a door that won’t latch where the two buildings meet, or a step in the floor. Designers handle this in different ways, from tying the foundations together to keeping them separate with a planned joint. Which one fits depends on the soil, the existing foundation type, and how the roof and walls connect. This is where it pays to coordinate with licensed engineers as needed, rather than letting the framing crew decide on the day.
Water around a new unit
A new ADU changes how water moves on your lot. There’s a new roof dumping rain, a new walkway, maybe a new patch of lawn or a planter bed right against the wall. The residential code asks for the ground to fall at least six inches within the first ten feet away from a foundation, or for drains or swales to move the water where lot lines don’t allow that.
On clay, steady and even moisture around the foundation matters as much as the slope. A sprinkler soaking one side of a new unit while the other side bakes is an invitation for uneven movement. Our companion post on sprinklers and slab edges goes deeper on that.
Questions to settle early
Before you sign a design contract, it helps to have answers to these:
- Is there an existing soils or as-graded report for the lot, and what does it say about expansion and fill?
- Is the house on a conventional slab, a raised floor, or a post-tension slab?
- For a garage conversion, is the slab level, dry, and free of offset cracks?
- Will the ADU be detached, attached, or a conversion, and how will it connect to the existing foundation?
- Where will roof water and irrigation go once the unit is built?
None of these is meant to talk you out of an ADU. They’re meant to keep the project from turning into a crack-repair job five years after move-in. Our foundation inspection page explains what a pre-design look at the existing house can cover.
What shapes the foundation scope
Every lot is different, and so is every budget. The factors that tend to move the foundation side of an ADU are:
- What the soils report finds about expansion and fill.
- Detached, attached, or conversion.
- Conventional slab, post-tension slab, or raised floor on the existing house.
- How much grading or soil replacement the site needs.
- Access for equipment into the backyard.
- Drainage changes the new unit requires.
- City plan review and permit requirements for the final design.
We serve the whole city, from the older blocks west of I-805 to the newest east-side villages. If you’re weighing an ADU and want a clear look at the ground and the existing foundation first, request a Free Quote. Bring any soils reports or plans you have. They make the first conversation much more useful.