Can You Put a Prefab Home on a Basement? [and the Alternatives]

Last updated on December 1, 2021

The answer is yes, a prefab home can be placed on the basement. This option provides an added space that can be used in many ways. It can be a safe place to shelter from the weather, too. And it adds to your home’s real estate value.

A modular home foundation is basically just like building a regular frame-on or stick-built home except for two key differences:

  1. The walls don’t come together until after the roof has been installed;
  2. There’s no need to pour concrete footings because there aren’t any permanent supports underneath the structure.

Instead, each module sits atop its own set of temporary support beams called jack posts. These jacks provide enough stability so that the modules themselves won’t have to bear much weight.

You can choose between a crawl space or a basement when planning for a foundation. It’s difficult to change your choice once the work is done because either one needs to be selected initially. Decide which type of foundation you want. This choice will impact the whole construction process. You need to think about the dimensions of the basement as well.

A modular home cannot be built on a slab foundation if there is space between the subfloor and the ground. Local building codes may have differing requirements for modular home foundations, but there are the most common ones.

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Foundation Over Basement

modular house on foundation

If your house were originally constructed over a crawlspace, it would not work properly when converted into a modular home. You’ll need to remove all existing materials before installing the new foundation. If this isn’t possible, you should contact a professional who specializes in removing old material safely.

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Jack Posts

Each module rests on top of four jack posts that serve as temporary supports while the rest of the construction process occurs. Jacks must be at least 16 inches deep below grade level. A minimum of three jack posts per side is required.

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Footers

Footing refers to the area beneath a foundation wall. In a conventional home, footings consist of poured concrete slabs that extend down to bedrock. When converting a traditional home into a modular one, footings become unnecessary since the entire structure doesn’t sit directly on the soil. However, some local regulations require them anyway. For example, California requires footings under certain conditions.

To meet these standards, builders often install large blocks of stone or other heavy objects around the site’s perimeter to create a stable base.

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Concrete Slabs

When constructing a modular home, the builder lays a series of plywood sheets across the site first. Then he places the modules onto those sheets using special connectors. Once everything is assembled, the builder pours concrete inside the framework. After the concrete is set up, the builder removes the plywood sheets and installs the exterior sheathing.

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Crawl Space

The unfinished area beneath a home is called a crawl space. The foundation doesn’t allow you to stand up, but it does provide strong support for your home and allows it to be raised off the ground. 

Crawl spaces can range from 4 to 12 square feet. Most people use an open-air crawl space instead of a closed system. This means that the dirt stays outside rather than being pumped through pipes. It also makes ventilation easier.

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On-frame vs. Off-frame Modular Home

On-frame modular homes: on-frame modular homes are typically made with steel frames and concrete floors. They’re usually manufactured at one location and then transported to another location for installation. These homes require more time and money to build, but they offer many benefits, including durability and strength.

Off-frame modular homes: off-frame modular homes are prefabricated before being shipped to their final destination. They have precast walls and roofs and may include windows and doors already installed. Some models even come fully furnished, so there’s no need to spend extra cash buying furniture.

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Prefab Home Foundation Requirements

The following information should help you understand what types of foundation systems are available for modular homes:

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Foundation Type

A concrete pad with poured footings is required if the house sits directly on top of the soil. If there is no soil underneath the structure, then a reinforced footing is needed. Reinforced footings consist of two layers of rebar embedded into cement blocks. These footings are usually 4 inches deep and 8 feet long.

If the building has a slab over its footprint, then only reinforcement is necessary. A slab is simply a thick layer of concrete placed over the frame. There is no requirement for additional reinforcing because the weight of the building rests solely upon this slab.

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Slope

Most foundations slope away from the front door towards the back of the property land. This helps prevent water from entering the home during rainstorms.

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Flooring System

There are three different options when installing a floor in a modular home. You can install hardwood or laminate flooring, carpet tiles, or vinyl plank flooring. Hardwoods such as oak and maple are very durable, while carpets tend to wear out quickly. Vinyl planks last longer than either option but aren’t nearly as attractive. Carpet tiles look like real wood and are easy to clean.

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Energy Efficiency

You’ll want to make sure that any new construction meets current energy efficiency standards. Energy-efficient features will save homeowners thousands of dollars each year by reducing heating costs and lowering utility bills. For example, most manufacturers now offer low E glass, which reduces heat loss through windows. Low E glazing uses less electricity and saves homeowners hundreds of dollars per month.

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Design Limitations

Some design limitations apply to modular homes. The size of your lot must be large enough to accommodate the modules. Also, some areas of town don’t allow modular homes due to zoning restrictions. Check with your local planning department to see whether these rules exist where you live.

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Additional Expenses

Depending on the type of foundation system used, it could cost anywhere between $1,000 – $10,000 to construct. In addition, you might also pay an architect fee depending on how complex the project is.

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Which Type of Foundation Will Work Best for Your Project?

It depends on where you live. In some areas, the soil under your lot isn’t suitable for pouring footings. For example, clay soils don’t hold enough moisture to keep footings stable. Soil conditions vary greatly throughout the country. Consult local codes and regulations regarding how much fill material must be used around the site’s perimeter.

In other cases, the soil might not be too wet or dry. Wetter soils won’t drain properly unless you add drainage holes near the base of each module. Dryer soils could cause problems if you pour footings that extend below ground level. The bottom of the modules shouldn’t sit higher than 3/4 inch above grade.

In addition, it’s important to consider whether the area surrounding the construction site has been previously developed. Many cities now require developers to perform extensive studies before beginning any new development projects. These studies determine the amount of fill material that needs to be added to ensure adequate stability.

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How Much a Basement Under a Modular Home Costs?

The price of putting a basement under a modular home varies depending on how much room there is between the existing structure and the new foundation walls. In general, adding a story adds about $10 per square foot to the total construction costs. Adding another level increases those costs by around $20 per square foot. So, if you plan to put a second story on your modular home, expect to pay more than double what you would normally spend for a single-story addition.

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Prefab Home Foundation Building Process

The process of constructing a modular home begins at the factory before shipping out to the job site. Modules arrive at the job site already assembled and ready to go into place. Once they’re set down onto the concrete pad, the modules are connected to form walls and floors. After the entire structure is complete, the roof trusses are installed, and then the exterior sheathing is attached. Finally, the interior finishes are applied.

After the steel frame is complete, concrete footings are poured into place around the home’s perimeter. These footings serve as anchors for the house and help prevent movement of the home over time.

A base plate is then attached to each footing before pouring the concrete pad. After the pads are set, they’re leveled and compacted to create a flat surface upon which the home sits.

Finally, after the concrete pads are finished, the walls and roof trusses are installed. This process takes anywhere from two weeks to several months, depending on the size of the building.

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Foundation Materials

When choosing materials for your modular home, look for ones with high compressive strength. Compression tests measure the ability of a material to resist being crushed when placed in compression. High-strength composite lumber meets this requirement. HSCs have excellent resistance to warping and twisting, making them ideal for use in foundations. They also provide superior thermal insulation properties compared to wood products.

Another option is reinforced concrete block. Concrete blocks come in many sizes and shapes.

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Modular Home Connection to the Foundation

Modular homes can have different foundations, but most are poured concrete slabs or precast piers with footings attached. Some modular homes use steel pilings driven into the ground as their primary support system. Steel piles provide more stability than wood posts because they don’t rot as wooden supports do. They also allow for greater loads to be placed in them. Poured-in-place concrete slabs are usually used when there isn’t enough room under the house for a full footing.

The modular home is delivered to the site after it’s been built. The modular home sections are connected to the modular home foundation. A final inspection will ensure the modular home foundation and site connections for utility connections are correct.

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FAQ

Can you put a prefab home anywhere?

Yes! Prefabricated buildings can be constructed almost anywhere. There are no zoning restrictions that apply to these structures. However, some local codes may require permits for certain features such as fireplaces, chimneys, etc. Check with your city/county planning department for details.

What kind of soil should I choose for my foundation?

Soil type plays a big role in determining whether a particular area has adequate moisture content. If you live near an ocean, consider using sand instead of clay soils. Sand absorbs water quickly so it won’t retain too much moisture. Clay soils tend to hold onto excess moisture longer. You’ll need to test the soil composition by digging up small samples and testing them for moisture content.

Are modular homes built on a foundation?

No. Modular homes are not built on a permanent foundation; rather, they’re assembled at the construction site. After the modules arrive at the job site, they’re bolted together. Once all the components are complete, the structure is lifted off its transport trailer and set down on a temporary base.

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