One contractor walks your crawl space and talks about bolts and plywood. Another looks at the same space and talks about a post that has sunk and a girder that has rotted at one end. A home inspector’s report mentions both. It is easy to come away thinking these are two names for the same job, or that whoever shows up first can handle all of it.
They are different jobs. A seismic retrofit prepares a house for sideways shaking it hasn’t felt yet. Raised foundation repair fixes supports that are already failing to carry the house’s weight. In an older raised-floor house, both happen in the same few feet of crawl space, and that is where the confusion starts.
Foundation repair fixes how the house carries its weight to the soil. A seismic retrofit improves how the house holds onto its foundation when the ground moves sideways. One does not substitute for the other, but each can depend on the other.
Two different questions
Foundation repair answers a gravity question. The California Residential Code says a foundation must be “capable of accommodating all loads” and of “transmitting the resulting loads to the supporting soil” (CRC R401.2). When a footing settles, a pad sinks, a post decays, or a stem wall cracks and moves, that job isn’t being done. Repair restores it.
A seismic retrofit answers an earthquake question. California’s prescriptive standard for this work on older wood-frame houses is Appendix Chapter A3 of the California Existing Building Code. Its stated purpose is “reducing the risk of earthquake-induced damage to existing wood-frame residential buildings.” It is careful about what that means: the provisions are “prescriptive minimum standards intended to improve the seismic performance of residential buildings; however, they will not necessarily prevent earthquake damage” (CEBC §A301.1). The City of San Diego has adopted Chapter A3 (SDMC §1411.0107).
Gravity pushes down every day. Earthquake forces arrive suddenly and push sideways. Our post on load path basics covers the downward path; the sideways path runs through different connections.
Side by side
| Foundation repair | Seismic retrofit (Chapter A3 type) | |
|---|---|---|
| Problem it addresses | Settlement, cracking, decay, or movement in the supports that carry the house’s weight | A house sliding off its foundation or short crawl-space walls racking in an earthquake |
| What prompts it | Sloping floors, sticking doors, cracks, a soft or leaning post, an inspection finding | Age of the house, a raised floor with no visible anchorage or bracing, a sale, insurance, or the owner’s own choice |
| Typical work | Footing or pad repair, post and girder replacement, stem wall repair, underpinning | Anchoring the sill plate to the foundation, bracing cripple walls, connecting floor framing to the sill or wall below |
| Where the rules live | CRC Chapter 4 for new foundation work; CEBC Chapter 4 for repairs | CEBC Appendix A3, an accepted standard plan set, or an engineered design |
| Who designs it | In-kind repair or an engineered design, depending on the problem | Prescriptive plans where the house fits Chapter A3; a registered design professional where it doesn’t |
The vocabulary for the retrofit side (cripple walls, sill plates, bolt-only versus brace-and-bolt) is in our posts on cripple wall bracing basics and the bolt-and-brace system. We won’t repeat it here.
Where the two jobs overlap
This is the part that’s easy to miss. Chapter A3 is a seismic standard, but it treats several foundation conditions as seismic weaknesses in their own right.
Chapter A3 counts some foundations as weaknesses
The code lists six conditions it considers structural weaknesses (CEBC §A303.1). Four are about the foundation or how the house connects to it:
- “Sill plates or floor framing that are supported directly on the ground without a foundation system that conforms to the building code”
- “A perimeter foundation system that is constructed only of wood posts supported on isolated pad footings”
- “Perimeter foundation systems that are not continuous” (with narrow exceptions, such as a short single-story wall extension or a porch)
- “A perimeter foundation system that is constructed of unreinforced masonry or stone”
The other two are unanchored sill plates and unbraced cripple walls, the two items a brace-and-bolt retrofit is known for.
For the first two conditions, the code doesn’t ask for bolts. It requires a new perimeter foundation (CEBC §A304.2.1). For partial perimeter foundations or unreinforced masonry foundations, it calls for evaluation “by a registered design professional,” who may recommend strengthening instead of replacement where the building official approves (CEBC §A304.2.2). In those houses, the seismic retrofit is foundation work.
The California Residential Mitigation Program says the same thing in plain terms for post-and-pier houses: “This type of home requires a new, continuous concrete footing that can be installed by a contractor using prescriptive (pre-engineered) plans. Or, an engineer may design an alternate foundation system for the house.”
A retrofit needs sound wood and sound concrete
Chapter A3 won’t let new hardware be fastened to damaged material. Existing wood that becomes part of the strengthening “shall be in a sound condition,” and “any wood material found to contain fungus infection shall be removed and replaced with new material” (CEBC §A304.1.2). A decayed sill or rotted cripple studs have to be dealt with before the retrofit can count on them.
Concrete matters too. When anchors are added to an existing foundation, “expansion anchors shall not be used where the installation causes surface cracking of the foundation wall at the locations of the anchor” (CEBC §A304.3.1). A stem wall that is crumbling or badly cracked changes what an anchor can do, and sometimes what anchor type can be used at all.
Foundation condition affects earthquake performance
FEMA’s Simplified Seismic Assessment of Detached, Single-Family, Wood-Frame Dwellings (FEMA P-50) includes foundation condition in its scoring for this reason. It notes that “a concrete foundation weakened by deterioration is more likely than a strong foundation to be damaged during an earthquake.”
It is also careful about cracks. “Cracks in foundation elements are not uncommon and can occur simply from normal shrinkage of concrete,” the guide says. Where cracking is extensive and settlement is significant, the soil conditions behind that movement “can also contribute to earthquake-induced deformations,” and “a foundation cracked prior to the earthquake is moderately more vulnerable to further cracking and movement during the earthquake.” P-50 is a FEMA assessment method, not a City of San Diego requirement, but the point carries: an unrepaired foundation problem can weaken the retrofit.
What foundation repair does not do
Fixing a settled footing or replacing decayed posts restores the house’s ability to carry its weight. It doesn’t anchor the sill or brace the cripple walls unless that work is added to the scope.
The existing-building code is direct about this. For damage below its “substantial structural damage” threshold, damaged elements “shall be permitted to be restored to their predamage condition” (CEBC §405.2.1). Even where damage to the lateral system is substantial, the code includes an exception: “One- and two-family dwellings need not be evaluated or retrofitted for load combinations that include earthquake effects” (CEBC §405.2.3). In plain terms, a foundation repair on a house generally does not carry a built-in seismic upgrade. If you want one, it has to be asked for. Our post on how inspections inform repair scope explains the repair thresholds.
What a seismic retrofit does not do
A crawl-space retrofit doesn’t stop settlement, dry out a wet crawl space, fix drainage, or level a floor. If your hallway slopes because a pad has sunk, new anchor bolts won’t change that.
It also has limits on which houses it fits:
- Slab houses. Chapter A3 doesn’t apply to “buildings or portions thereof constructed on concrete slabs on grade” (CEBC §A301.2). The California Earthquake Authority describes slab houses as “already more earthquake resistant.”
- Tall, stepped, or sloped crawl spaces. Cripple walls over 4 feet, and houses with poles or columns embedded in the ground, need analysis by a registered design professional (CEBC §A301.2). CRMP adds that sloped lots and stepped walls call for a licensed structural or civil engineer. On hillside lots, our hillside foundation repair page covers the foundation side.
- Rooms over a garage. That is a different vulnerability with its own standard. See our post on soft-story seismic concepts.
Which comes first?
There’s no universal order, and sequencing is a judgment for whoever evaluates the house. A few principles follow from the code itself:
- If supports are failing, the retrofit has nothing sound to attach to. Chapter A3’s sound-wood rule and its anchor restrictions mean decay and badly damaged concrete get addressed before or as part of the retrofit.
- If the perimeter is posts on pads or isn’t continuous, the foundation work is the retrofit. A new perimeter foundation comes first, and the anchorage is built into it.
- If the foundation is sound and continuous, the two can be separate projects. A house with a solid stem wall and an unanchored sill may need only the retrofit. A house with one sunken interior pad and good anchorage may need only the repair.
- Phasing is allowed. Chapter A3 says strengthening work “may be completed in phases” when the enforcing agency approves (CEBC §A304.6).
The insurance side follows the same logic. For houses with post-and-pier, post-and-beam, or unreinforced masonry foundations, the CEA’s retrofit discount requires “foundations that are placed continuously under all exterior bearing walls” and anchored to that perimeter foundation. Ask your insurer about current terms; we don’t speak for any insurer.
Questions to ask before you sign either scope
- Does the proposal name its basis: Chapter A3, a standard plan set the City accepts, or an engineered design?
- If cripple walls exist, does the retrofit address both anchorage and bracing?
- What happens if decayed sill plates or studs turn up once work starts?
- Is any foundation work part of the retrofit, or a separate scope with its own permit?
- For a foundation repair, does it leave the sill connection untouched, and is that what you want?
- Who inspects? Chapter A3 work is “subject to inspection by the code official” (CEBC §A304.5), and in San Diego altering or repairing a structure generally requires a Building Permit (SDMC §129.0202).
Our post on what drives foundation repair cost helps when comparing proposals.
Cost factors, not prices
We don’t publish prices. What moves the scope of combined seismic and foundation work: whether the perimeter foundation is continuous, the height and length of cripple walls, crawl-space access and clearance, how much decayed wood has to be replaced, the condition of the existing concrete, whether a standard plan fits or engineering is required, slope and hillside conditions, and permit or historic review requirements.
Where we fit
We evaluate raised foundations and seismic connections together, so the bid you get reflects both what is holding the house up and what is holding it on. Our raised foundation repair and seismic retrofit pages describe that work, and our foundation repair page covers other foundation types. When the house falls outside Chapter A3 or a foundation needs engineered design, we coordinate with licensed engineers as needed; our structural engineer page explains how. This post is general education, not engineering or legal advice about any specific house.
If an evaluation is your next step, our companion post on preparing for a seismic evaluation covers what to gather and what to ask.
Next step
If your crawl space shows a sunken post, a decayed sill, or no visible anchorage, or you simply don’t know which job your house needs, request a Free Quote for an on-site evaluation anywhere in La Jolla, or call (650) 739-6537.
Sources (accessed October 17, 2026)
- 2025 California Existing Building Code, Appendix Chapter A3, Sections A301.1, A301.2, A303.1, A304.1.2, A304.2.1, A304.2.2, A304.3.1, A304.5, and A304.6
- 2025 California Existing Building Code, Sections 405.2.1 and 405.2.3
- 2025 California Residential Code, Section R401.2
- San Diego Municipal Code §1411.0107 (adoption of CEBC Appendix Chapter A3) and §129.0202
- FEMA P-50, Simplified Seismic Assessment of Detached, Single-Family, Wood-Frame Dwellings (May 2012)
- California Residential Mitigation Program, Raised Foundation House Retrofit
- California Earthquake Authority, Get a Discount (premium discount eligibility)