Sometimes yes, sometimes no, and the answer is decided by three things that have nothing to do with your design: what the ground under the house is doing, what the state has mapped your parcel as, and how strict your local building department has chosen to be. The frustrating part is that the third one can override the first two, and it varies by jurisdiction.
RDD Solutions is a residential design and drafting firm. We do not build, we are not an architecture firm, and we do not produce geotechnical reports — that work belongs to a different license. We draw the plan sets a geotechnical engineer’s recommendations get built into, which means we spend a lot of time on whether a report is coming and when.
What a Geotechnical Report Is, and Who Produces It
A geotechnical report, often called a soils report, is a site-specific investigation of the ground your building will sit on. It is produced by a licensed geotechnical engineer or a California Certified Engineering Geologist. Not by a designer, not by a drafting firm, not by your contractor.
The investigation is physical: borings, test pits or probes to bring up samples, laboratory testing, a review of published geologic mapping and prior nearby investigations, and a walk of the site by someone trained to read what a slope is telling them. What comes back is a report with findings and, more importantly, recommendations — what foundation type the site will support, what bearing capacity to design to, how deep footings need to go, how to handle drainage and grading, whether retaining is needed and what it has to resist, and what seismic design parameters apply to that parcel.
Those recommendations are not advisory. Once a report exists, the design has to follow it, and the building department will check that it does.
What Typically Triggers One
No single condition decides it. These are the ones that come up repeatedly:
- Slope. The steeper the ground, the more the question shifts from “will it hold the weight” to “will it stay where it is.” Slope stability, surcharge on downhill structures and drainage become design problems rather than assumptions.
- Mapped seismic hazard zones. If the state has mapped your parcel inside a liquefaction or landslide Zone of Required Investigation, the ground is already flagged as potentially weak.
- Expansive soils. Clays that swell when wet and shrink when dry move foundations seasonally, and foundation design has to answer that specifically.
- Retaining walls. A retaining wall holds back a mass of soil. What that soil weighs and how it behaves saturated is a geotechnical question, and height plus surcharge is usually what pushes a wall over a threshold.
- New construction versus additions. A new house on an undeveloped hillside is the strongest trigger there is. Additions are treated more variably, and size is often the deciding factor — small, single-story work on level, undisturbed ground is where waivers tend to live; work adding substantial load, a second story or a new foundation is where they tend not to.
- Prior site disturbance. Fill of unknown origin, an old cut or pad, a previous grading permit, or evidence of past movement all invite investigation no matter how settled the lot looks now.
Seismic Hazard Zones and the Exemption Most Homeowners Never Hear About
This is where homeowners get incorrect information most often, so it is worth reading twice. Under California’s Seismic Hazards Mapping Act, the state maps areas as Zones of Required Investigation for liquefaction and for earthquake-induced landslides. A mapped zone means the state has determined that weak soil or rock may be present, and within one a site-specific investigation by a certified engineering geologist or civil engineer is required before a development permit is issued.
Here is the exemption. Single-family wood-frame or steel-frame dwellings up to two stories, not part of a development of four or more units, are exempt from that state requirement. A second exemption matters just as much for remodels: alterations or additions within a seismic hazard zone that do not exceed either 50% of the value of the structure or 50% of the existing floor area are also excluded. Which describes a very large share of the houses people are remodeling and adding to.
Now the catch. That exemption is from the state requirement. Local jurisdictions may impose stricter standards, and many do — Monterey County among them. A homeowner who reads the state exemption and concludes they are in the clear can be entirely right about state law and entirely wrong about what their building department will ask for. The state sets a floor, not a ceiling.
Zone status carries one more consequence homeowners find genuinely useful: a parcel in a mapped zone triggers Natural Hazard Disclosure when the property is sold. That matters whether or not you are building, and if you are buying a hillside property with a remodel in mind, the disclosure package is one of the first places to look for what you are walking into.
Worked Example: Monterey County’s Published Thresholds
Monterey County illustrates what “stricter locally” looks like in practice. Its published policy is that a geotechnical investigation is generally required, on the basis of the county’s high seismic setting and the need to evaluate slope instability, liquefaction, loss of soil strength and differential settlement. That is a default of yes with exceptions, rather than a default of no.
The building official may waive a full investigation where soils conditions are already known from prior nearby investigations. As published, the situations that may avoid a full report include:
- one-story additions of 500 square feet or less on level, undisturbed grade
- second-story additions of 500 square feet or less with engineering design
- detached accessory structures of 500 square feet or less
- fences under 8 feet, and decks 4 feet or less above grade
- retaining walls with 5 feet or less of level surcharge
And the condition that matters most on a hillside: slopes of 3:1 or steeper require sign-off by a California Certified Engineering Geologist or a registered geotechnical engineer. Three feet of horizontal run per foot of rise is not a dramatic slope, and plenty of lots that read as gently sloped meet it somewhere across the building area.
Two cautions. These are a waiver from a full investigation, not a guarantee of no involvement — the building official decides, leaning on prior nearby data. And they are the thresholds as published, on a county page that cites the 2022 California Building Code. The 2025 California Building Standards Code took effect January 1, 2026, and we have not confirmed the policy has been updated for it, so treat the numbers as a guide to how the county thinks rather than a current citation.
Requirements and timelines described here were researched in September 2026 and are typical rather than guaranteed. Verify current requirements with the building department for your jurisdiction.
The same logic lands differently depending on where you are. Carmel Valley work runs through the Carmel Valley Master Plan, with design review, viewshed restrictions and oak tree provisions layered on top of the county’s soils expectations. Pebble Beach is unincorporated county in the Del Monte Forest, carrying dual approval through both the Pebble Beach Company Architectural Review Board and Monterey County, so a sloped, treed lot there answers to two review paths at once. Inside the City of Monterey, a slope over 15% triggers architectural review on its own, independent of anything geotechnical.
What a Report Costs You in Time and Sequence
The schedule impact is less about the report’s own turnaround than about where it sits in the order of operations, and that is what catches people.
The design has to be far enough along that the geotechnical engineer knows where the building is going and roughly what it weighs, because they are investigating a footprint, not a parcel in the abstract. Field work follows, then laboratory testing, then the written report. The structural engineer designs the foundation to its recommendations, and only then do those details become drawings that can go into a permit set. If the report calls for a different foundation approach than the design assumed — deepened footings, piers, a different retaining strategy — that flows backward into the drawings and sometimes into the design.
So commission it early. A report started at the beginning runs in parallel with design development; one started when the permit set is nearly finished stops everything and can force rework. Weather matters too: access for drilling equipment on a steep, wet lot is not available on every day you might want it.
How the Findings Reach the Drawings
A geotechnical report is not filed and forgotten. It becomes design input, and it is visible all over a permit set. Foundation type and depth come straight from it. Allowable bearing pressure sets footing sizes. Site seismic design parameters feed the structural engineer’s lateral calculations. Drainage and grading recommendations show up on the site plan and in the details. Retaining wall design responds to the soil properties and surcharge the report establishes. Where it calls for special inspection or observation during construction, that gets noted on the drawings so it is built into the job rather than discovered at inspection.
This is why the report has to precede construction documents rather than accompany them. On a home addition, the foundation is where new structure meets both the ground and the existing building, and both relationships are shaped by what the soil will carry. On a second-story addition the question sharpens: new load travels down through the existing structure into a foundation designed for a different building. On a whole-house project or new construction on a hillside lot, the report is often the first thing commissioned, because so much of the design depends on the answer.
How to Find Out Where You Stand
Three steps, and none of them require hiring anybody. First, find out whether your parcel sits inside a mapped Zone of Required Investigation for liquefaction or landslide; if you bought recently, the Natural Hazard Disclosure from the sale will tell you. Second, look at the slope across the actual building area rather than the lot as a whole, because a 3:1 threshold is met by more lots than people expect. Third, ask your building department directly what they require for the scope you have in mind — local practice is what decides this, and a phone call beats any amount of reading. Our post on California building permit requirements covers what else comes attached to a permit application.
Geotechnical and Soils Report FAQs
Do I need a soils report for a home addition?
It depends on your jurisdiction, the slope, and the size and type of the addition. Monterey County, as an example, generally expects a geotechnical investigation given the region’s seismic setting, but the building official may waive a full report in defined situations — as published, those include one-story additions of 500 square feet or less on level, undisturbed grade, and second-story additions of 500 square feet or less with engineering design. Those thresholds cite the 2022 code, so confirm current requirements with the department before planning around them.
Who can produce a geotechnical report?
A licensed geotechnical engineer or a California Certified Engineering Geologist. A design and drafting firm cannot produce one, and neither can a contractor. We coordinate with the geotechnical engineer and build their recommendations into the plan set, but the investigation and the report are a separate professional engagement you contract directly.
My house is a single-family home. Aren’t I exempt?
From the state requirement, often yes. Under the Seismic Hazards Mapping Act, single-family wood-frame or steel-frame dwellings up to two stories that are not part of a development of four or more units are exempt from the state’s site-investigation requirement inside a mapped Zone of Required Investigation. But local jurisdictions may impose stricter standards, and Monterey County does. The state exemption is a floor, not a ceiling, so it does not answer the question on its own.
What counts as a steep enough slope to trigger a report?
There is no single statewide number; it is set locally. In the Monterey County example, slopes of 3:1 or steeper require sign-off by a California Certified Engineering Geologist or a registered geotechnical engineer. That is three feet of horizontal run per foot of rise, a shallower slope than most people picture, and it is assessed across the building area rather than averaged over the whole lot.
What does a mapped seismic hazard zone mean if I am not building?
It means the state has identified that weak soil or rock may be present on or near your parcel, and it triggers Natural Hazard Disclosure when the property is sold. That disclosure is also the easiest way to find your zone status if you bought recently — it will be in the paperwork from the sale, and it is worth checking before buying a hillside property you intend to remodel.
Planning a Hillside Project?
Tell us where the lot is and what you are hoping to build, and we will tell you what the site conditions are likely to mean for the drawings, the sequence and who else needs to be on the team. Get in touch.