You have finally decided to add that bedroom, sunroom, or bigger kitchen. Before you pick tile or paint colors, though, the ground under your project deserves your attention, and building on expansive soil should top your list.
Building on expansive soil is a fact of life across much of Colorado, and the clay beneath your yard can move enough to crack foundations, jam doors, and buckle floors. The good news is that a little planning up front lets you build an addition that stays solid for decades.
Many Colorado homeowners first learn about swelling soil when a basement floor heaves or a drywall crack keeps coming back. The Colorado Geological Survey calls swelling soil a common and sometimes serious geologic hazard in Colorado, capable of causing severe damage to houses. The survey also notes that damage can build slowly over time, which explains why one house on your street may show problems years before the house next door.
As a senior home designer, I have worked on many additions along the Front Range where the soils report shaped the entire project. In this guide, you will learn what expansive soil is, why bentonite clay swells and shrinks, what a soils report tells you, and how to choose a foundation for your addition. You will also learn the water management habits that protect your investment long after construction ends.
Expansive soil is clay-rich ground that changes volume as moisture levels change. When the clay absorbs water, the soil swells and pushes upward and outward. When the clay dries, the soil shrinks and can leave gaps under footings and slabs. This repeated swelling and shrinking creates pressure that can lift a lightweight structure, crack concrete, and twist framing out of square.
Engineers often call these soils swelling soils, shrink-swell soils, or expansive clays. The amount of movement depends on the type of clay, how much clay the soil contains, how dry the soil is before water arrives, and how deep the moisture reaches. For anyone building on expansive soil, a small change in moisture can produce a surprisingly large change in volume, which is why the work requires special care rather than a standard foundation design copied from a flatter, sandier region.
Colorado’s geology explains why building on expansive soil comes up so often here. Much of the Front Range urban corridor sits on or near bedrock formations, such as the Pierre Shale and the Denver Formation, that contain layers of clay and claystone. Over millions of years, volcanic ash settled into ancient seas and lakes and weathered into clay minerals. Those layers now sit at or near the surface in many neighborhoods from Fort Collins through Denver to Colorado Springs and Pueblo.
Our climate makes things worse. Colorado’s semi-arid weather keeps much of the soil fairly dry for long stretches. When lawn irrigation, a leaking gutter, or a wet spring adds water to dry clay, the soil has a large capacity to swell. That pattern of dry soil followed by new moisture explains why building on expansive soil in Colorado can produce more movement than you might see in wetter parts of the country.
Bentonite is a clay formed from weathered volcanic ash, and the mineral responsible for most of the swelling is montmorillonite. Montmorillonite has a layered structure that pulls water molecules between the layers, much like a stack of paper soaking up a spill. As water enters, the layers spread apart and the clay expands. Some bentonite-rich soils can swell to many times their dry volume, and the pressure from that swelling can exceed the weight of a typical house.
That pressure matters when you are building on expansive soil for an addition. A one-story room addition weighs far less than the upward force swelling clay can generate, so a lightweight foundation may simply ride up and down with the soil. Meanwhile, your existing house may sit on a different foundation system that moves differently or not at all. When the two structures move independently, cracks appear where the new walls meet the old ones.
You might also hear about the opposite problem. During drought or when a large tree pulls moisture from the ground, clay can shrink and leave a footing partially unsupported. Both swelling and shrinking cause damage, so successful building on expansive soil focuses on keeping moisture levels steady and isolating the structure from soil movement.
Before building on expansive soil for a new addition, take a walk around your current home with fresh eyes. Your existing house can tell you a great deal about the soil beneath your lot. Look for diagonal cracks above doors and windows, doors that stick or will not latch, cracks in basement walls or slabs, gaps between trim and walls, and patios or sidewalks that tilt toward the house.
None of these signs automatically means you have a serious problem, since some cracking happens in every house as concrete cures and lumber dries. Still, a pattern of recurring or widening cracks deserves attention. Take photos, note the dates, and share them with your designer and engineer. That history helps your team understand how your soil behaves and how your new foundation should respond.
A soils report, also called a geotechnical report, is the single most important document for building on expansive soil. A licensed geotechnical engineer drills test borings on your lot, collects samples, and runs laboratory tests to measure moisture content, density, and swell potential. The report then explains what the engineer found and recommends a foundation type, design pressures, drainage measures, and construction methods.
Colorado takes soil disclosure seriously, which reflects how common building on expansive soil is here. Under Colorado Revised Statutes section 6-6.5-101, builders and developers must give buyers of new homes a summary of the soil analysis and site recommendations at least 14 days before closing. Where expansive soils are recognized, builders must also provide a publication about soil problems, construction methods, and maintenance. If you bought a newer home, look through your closing documents, because an original soils report may already sit in your files.
An older soils report gives useful background, but most designers and building departments want current information for an addition. Soil conditions can change after years of irrigation, new plantings, and drainage changes. In addition, the original report may not have tested the exact spot where your new room will go.
Soils reports use technical language, but a few sections matter most to you. The boring logs describe each soil layer and the depth to bedrock or groundwater. Swell test results show how much a sample expanded when wetted under a set load, often expressed as a percentage. The recommendations section tells your designer and structural engineer which foundation type to use, how deep to go, and what drainage and backfill details to include.
Ask your engineer to walk you through the findings in plain terms. A good engineer will explain whether your soil has low, moderate, or high swell potential and what that means for cost. Understanding the report helps you make informed decisions rather than treating the foundation as a mystery line item on your budget.
The International Residential Code, which most Colorado cities and counties adopt with local amendments, addresses expansive soil directly. The code allows building officials to require soil tests where expansive soils are likely, and the code requires foundations on expansive soil to be designed to resist the movement. For building on expansive soil, many Colorado jurisdictions go further and require a soils report or an engineered foundation for any addition. Check with your local building department early, since requirements vary from one city to the next.
Once you have a soils report, your structural engineer will design a foundation that matches the conditions. No single system works best everywhere. The right choice depends on swell potential, depth to bedrock, the design of your existing foundation, your budget, and whether your addition includes a basement or crawlspace.
Most foundation strategies for building on expansive soil follow one of three ideas. You can reach past the active clay to stable material below, you can remove and replace the problem soil, or you can design the structure to tolerate some movement without damage. The most common options in Colorado include the following:
Each option carries trade-offs in cost, schedule, and performance, so let your soils report and structural engineer guide the decision. Drilled piers often cost more but offer strong protection on high-swell sites. Over-excavation works well when bedrock sits too deep for practical piers. A structural floor costs more than a basic slab but prevents the floor heave that plagues many Colorado basements.
Additions bring a unique challenge when building on expansive soil. Your new foundation must work alongside your existing one, and the two may respond to the soil in different ways. If your house sits on drilled piers and your addition sits on shallow footings, the addition may move while the house stays put. That difference creates cracks and leaks where the old and new roofs and walls connect.
Your engineer has several ways to handle this. In many cases, the best solution matches the addition foundation to the existing foundation type. In other cases, the engineer designs a structural connection between the two, or intentionally separates them with a flexible joint so each can move without damaging the other. In our projects, we review the existing foundation plans, when available, before we sketch the addition layout, because that information can influence where the new room should go.
If your addition includes a basement, the floor deserves special attention when building on expansive soil. Many Colorado basements use a floating slab, which rests on the soil and can move up and down independently of the foundation walls. Builders then frame interior walls with slip joints, a gap at the bottom or top of the wall that lets the slab rise without lifting the framing above. Without that detail, a heaving slab can crack drywall, jam doors, and damage plumbing on the floor above.
A structural floor eliminates most of that risk by suspending the floor above the soil, usually over a crawlspace. The cost is higher, but many homeowners find the peace of mind worthwhile, especially on sites with high swell potential. Your soils report will often state whether a slab-on-grade floor carries acceptable risk for your site.
Even the best foundation performs better when you keep water away from the soil around your home. Since moisture changes drive swelling, consistent drainage remains the most effective long-term strategy when building on expansive soil. Good drainage protects your new addition and your original house at the same time.
Start with grading. The International Residential Code calls for the ground to slope away from foundations, with a fall of at least 6 inches within the first 10 feet where site conditions allow. Add gutters to every roof edge and extend downspouts several feet away from the foundation, discharging onto splash blocks or into drain lines that carry water well beyond the backfill zone. Many soils reports include specific distances, so follow those recommendations exactly.
Yard and planting choices matter as well. Avoid planting thirsty lawns or flower beds right against your foundation, and keep sprinkler heads aimed away from walls. Large trees planted too close to the house can dry out the soil and cause shrinkage, so plan new trees at a safe distance. A perimeter drain system, often called a foundation drain, may also be recommended to collect groundwater around a basement and route the water to a sump pit or daylight outlet.
Building on expansive soil goes smoothly when you follow a sensible order. First, gather any existing documents, including your original soils report, foundation plans, and closing papers. Next, hire a geotechnical engineer to test the addition site. Then work with your designer and structural engineer to choose a foundation system and lay out the addition around those recommendations.
Budget for the foundation honestly, because building on expansive soil costs more below grade than many homeowners expect. On high-swell sites, the foundation can represent a larger share of your addition cost than you might expect, and cutting corners here can lead to repairs that cost far more later. Ask your contractor for line-item pricing on the foundation, drainage, and backfill work so you understand where your money goes.
Finally, protect the site during construction. Open excavations exposed to rain or snowmelt can soak the clay and change conditions before concrete goes in. A good contractor covers excavations, pumps out standing water, and schedules inspections promptly. Your geotechnical engineer may also inspect pier holes or footing excavations before concrete placement, which adds a layer of quality control worth every penny.
Colorado’s clay soils do not have to stand between you and the addition you want. With a current soils report, a foundation designed for your site, and careful water management, you can build an addition that stays level, dry, and comfortable for decades. Building on expansive soil simply asks you to respect the ground beneath your home and plan accordingly.
Start with knowledge, not guesswork. Look for warning signs around your existing house, gather your records, and bring in a geotechnical engineer before you finalize your design. When you approach building on expansive soil with the right team and the right information, your new space can become a lasting part of your home rather than a source of cracks and worry.
Before building on expansive soil, look for clues. Recurring cracks, sticking doors, and heaving basement floors often point to expansive soil. Your original soils report, if you have one, will describe your soil type. For certainty, hire a geotechnical engineer to drill test borings and run swell tests where you plan to build.
Many Colorado building departments require a soils report or an engineered foundation for additions when building on expansive soil, especially in areas known for swelling soil. Even when not required, a soils report is a wise investment, because the foundation recommendations protect your entire project.
No single foundation fits every site. Drilled piers to bedrock, over-excavation with structural fill, structural floors over crawlspaces, and post-tensioned slabs all perform well when matched to the right conditions. Your geotechnical and structural engineers will recommend the best option for your lot.
Yes, in many cases. Basement additions on expansive soil often use drilled piers or engineered footings, a floating slab with slip joints in interior walls, or a structural floor. Proper drainage around the foundation walls also matters.
Keep water away from your foundation. Maintain grading that slopes away from the house, clean gutters regularly, extend downspouts, and avoid watering plants right next to your walls. These simple habits reduce moisture swings in the soil.
Whether you want a sunroom, a primary suite, or a full second story, Colorado Building Plans can help you design your home addition that works with your soil, not against your soil. We coordinate with geotechnical and structural engineers, create 3D renderings and permit-ready plans, and guide you through approval. Book a free call to talk through your project, or try our free Home Addition Planner to organize your ideas before we meet.