What to Do When Your Lot Is in a Liquefaction Zone

When the Ground Under Your Dream Home Could Turn to Liquid

You have found the perfect lot for your custom home. The location is right, the neighborhood checks every box, and you are ready to start planning. Then you pull up the California Geological Survey’s seismic hazard maps and discover your lot is in a liquefaction zone.

For many Bay Area homebuyers and landowners, this discovery raises immediate concern. Does this mean you cannot build? Will the house sink in an earthquake? Is the lot worthless?

The short answer is no, no, and no. Building on a liquefaction-prone lot is absolutely possible with proper engineering. But it does affect your foundation design, your budget, and your planning timeline. This guide explains what liquefaction is, how it affects construction, and exactly what steps to take.

What Liquefaction Is and Why It Matters

Liquefaction is a phenomenon where saturated, loosely packed soil temporarily loses its ability to support weight during an earthquake. When strong shaking occurs, the water pressure between soil particles increases until the particles separate and the soil behaves like a thick liquid rather than a solid. This effect can last from a few seconds to several minutes.

During liquefaction, the consequences for buildings can be severe:

• Settlement: Buildings sink into the softened ground, sometimes unevenly
• Lateral spreading: The ground moves horizontally, especially near slopes or free faces like creek banks
• Sand boils: Pressurized water and sand erupt from the ground surface
• Foundation failure: Spread footings lose bearing capacity as the soil beneath them liquefies

The 1989 Loma Prieta earthquake caused dramatic liquefaction damage in San Francisco’s Marina District, where homes built on fill material sank and tilted as the ground beneath them liquefied. This event remains one of the most vivid examples of liquefaction damage in U.S. history.

Where Liquefaction Zones Are in the Bay Area

The Bay Area has extensive areas of liquefaction risk. The California Geological Survey (CGS) publishes official Seismic Hazard Zone maps that identify areas where liquefaction and earthquake-induced landslides are potential hazards. These maps are the authoritative reference used by building departments.

High-Risk Areas

• San Francisco: The Marina District, SoMa, Mission Bay, Treasure Island, and much of the eastern waterfront are built on fill material placed over former marshland and bay mud
• East Bay shoreline: Parts of Oakland, Alameda, Berkeley, Emeryville, and Richmond sit on bay fill or alluvial deposits
• South Bay: Extensive areas of Milpitas, Sunnyvale, Santa Clara, and San Jose, particularly near creeks and the southern reaches of the bay
• Peninsula: Low-lying areas of Redwood City, Foster City (built entirely on fill), and sections of San Mateo

How to Check Your Lot

The CGS maintains an interactive online map viewer (maps.conservation.ca.gov) where you can look up any address and see whether it falls within a mapped seismic hazard zone. Your local building department will also have this information.

If your lot is in a mapped liquefaction zone, the building department will require a geotechnical investigation as part of the permit process. This is not optional.

What to Do Step by Step

Step 1: Get a Geotechnical Investigation

A geotechnical investigation is the foundation (pun intended) of everything that follows. A licensed geotechnical engineer will:

• Drill one or more soil borings on your lot, typically to a depth of 30 to 50 feet or until reaching stable soil or bedrock
• Collect soil samples at regular intervals
• Measure groundwater depth
• Perform laboratory tests on the samples to determine soil type, density, and liquefaction susceptibility
• Analyze the results using current seismic hazard data for your area
• Provide a written report with specific foundation recommendations

This investigation costs $3,000 to $8,000 for a typical residential lot. For larger or more complex sites, costs can reach $10,000 to $15,000. The report is submitted to the building department as part of your permit application.

Step 2: Understand the Findings

The geotechnical report will classify the liquefaction risk on your specific lot and provide recommendations. Not all lots within a mapped liquefaction zone have the same level of risk. Key factors include:

• Soil type: Loose, sandy soils are most susceptible. Dense or clayey soils may have lower risk even within a mapped zone
• Groundwater depth: Shallow groundwater (within 30 feet of the surface) increases liquefaction potential
• Depth to stable soil: This determines whether deep foundation elements can reach a stable bearing layer at a reasonable cost
• Estimated settlement: The report will estimate how much the ground could settle during a design-level earthquake

Step 3: Work with a Structural Engineer on Foundation Design

Armed with the geotechnical report, a structural engineer designs a foundation system that can handle the identified risks. Common foundation solutions for liquefaction zones include:

Drilled piers (caissons): Concrete-filled shafts drilled through the liquefiable soil to bedrock or a dense bearing layer below. This is the most common solution when stable soil exists at a reachable depth. Piers are typically 18 to 36 inches in diameter and can extend 20 to 60+ feet deep.

Mat foundation: A thick concrete slab (typically 18 to 30 inches) that covers the entire building footprint. The mat distributes loads over a large area, reducing the building’s sensitivity to localized settlement. This works well when liquefaction risk is moderate and expected settlement is relatively uniform.

Grade beams with piers: A hybrid approach where deep piers support a network of reinforced concrete beams that, in turn, support the building. This combines the deep support of piers with the load distribution of a continuous foundation system.

Soil improvement: Techniques like compaction grouting, vibro-compaction, or stone columns can densify the soil to reduce or eliminate liquefaction potential. This approach treats the problem at the source rather than engineering around it, but is only effective for certain soil types and conditions.

Step 4: Factor Foundation Costs into Your Budget

Liquefaction-appropriate foundations cost significantly more than standard residential foundations. Here are general cost ranges:

• Standard residential foundation (for comparison): $15,000-$40,000
• Thickened mat foundation for moderate risk: $35,000-$75,000
• Drilled pier foundation with grade beams: $50,000-$120,000
• Soil improvement plus foundation: $40,000-$100,000

The total added cost depends on the depth to stable soil, the number and size of piers required, and the overall building footprint. For a 2,500 to 4,000 square foot custom home, plan for an additional $20,000 to $100,000 above what a standard foundation would cost.

Step 5: Coordinate with the Building Department

Bay Area building departments in liquefaction zones are experienced with these projects. They will require:

• The geotechnical investigation report
• A structural engineer’s foundation design referencing the geotechnical recommendations
• Special inspection requirements during construction (a geotechnical engineer must observe key foundation work)
• Potentially, a peer review of the geotechnical report by the city’s consulting geotechnical firm (this adds $2,000-$5,000 in fees and 4-8 weeks to the permit timeline)

Build these requirements into your project timeline. Geotechnical work and peer review can add 2 to 4 months to your pre-construction schedule.

How to Prevent Costly Surprises

Investigate Before You Buy

If you are purchasing a lot for a custom home, check the CGS seismic hazard maps before making an offer. If the lot is in a mapped zone, you can make your offer contingent on a satisfactory geotechnical investigation. This protects you from purchasing land where foundation costs would make the project infeasible.

Budget Realistically from Day One

When building in a liquefaction zone, your foundation is not the place to look for cost savings. Work with your design-build team to establish a realistic budget that includes geotechnical investigation, engineering design, and construction costs for the appropriate foundation system.

Choose the Right Design-Build Team

Not every builder has experience with deep foundation work in liquefaction zones. Ask prospective builders about their experience with drilled pier foundations, mat foundations, and coordinating with geotechnical engineers on seismically challenging sites.

When to Call a Professional

If you own a lot in a mapped liquefaction zone and are considering building, engage a geotechnical engineer as your very first step. Do not finalize architectural plans, structural designs, or construction budgets without a geotechnical report in hand. The soil conditions on your lot will influence everything from foundation type to floor plan layout to total project cost.

If you already own a home in a liquefaction zone and are planning a major addition or remodel, a geotechnical investigation may also be required, especially if your project adds significant load to the existing foundation.

Why Custom Home Design and Build

Building a custom home in a liquefaction zone requires tight coordination between geotechnical engineers, structural engineers, architects, and the construction team. This is exactly where a design-build firm excels.

At Custom Home, we manage the entire process. We coordinate the geotechnical investigation during feasibility planning, before you commit to a design. Our structural engineer works directly with the geotechnical engineer to design a foundation system that satisfies code requirements and fits your budget. And our construction team has the experience and equipment relationships to execute complex foundation work on schedule.

Because we handle design and construction under one roof, there are no gaps between what the engineer designs and what the builder constructs. Every decision is informed by all disciplines, from the first soil boring to the final coat of paint.

If you are considering a custom home on a Bay Area lot and have questions about soil conditions, seismic hazards, or foundation requirements, we are here to help.

Schedule a consultation to discuss your lot and explore your options.