How To Build a Modular Home Foundation

Modular homes, or “prefab homes,” are a great way to build a new home and save up to 15%! Many modular home builders will offer general contractor services to go with the purchase of your home. The purpose of the general contractor is to do all of the site prep, running of utilities, foundation, and installation of the final assembly. Modular home builders must also supervise the lot preparation, contract a crane service and prepare the area from where the crane can operate. At Next Modular we call that our “Turn Key Service” and with that you don’t have to worry about any of those details. We take care of it all and hand you the keys when we’re done. 

Can I be my own General Contractor…?

But what if you want to act as your own general contractor? Well, unlike other modular home builders, with Next Modular, you can absolutely do that! You can actually save a bunch of money that way but it’s going to mean a fair bit of work for you as the homeowner. One of the first things you will have to deal with, after selecting one of our many modular homes for sale, is to prepare the site and foundation for your new modular home. 

The good news is that prefab homes are the same as any other site built home and don’t really require any special accommodations. Alternatively, single wide or double wide manufactured homes must be set on a HUD approved site which involves piers with crushed aggregate or a slab with Piers. Often they will require steel I-beams and special permitting to put them on your own property. With modular homes, you and your contractors don’t have to worry about any of that.

What’s my First Step When Doing my Own Foundation?

So let’s say you’ve purchased your property and you’ve selected one of our many modular homes for sale (browse all of our modular home floor plans here). The first thing you’ll need to do is to prepare the site for your foundation. Let’s look at a few steps you’ll need to take to get your foundation ready for the delivery of your new prefab home. 

1. Permits and Site Inspection

The first step is to acquire the required permits from the regional building department. This will include permits for things like zoning, building, and driveway permits. Depending on your soil conditions, you might also need to perform a soil test to determine what type of foundation options are available to you as well as septic options that you’ll need to consider later on in the construction process. 

2. Get a Survey

The second step is to order a detailed site inspection by a land surveyor. Have a surveyor clearly mark the boundaries of your land with flags to let your modular home builder and their workers know where they can and cannot work, but also where they can and cannot set up their equipment. You will probably need portable toilets on your construction site and your neighbors won’t be too happy if they’re placed on their land.

The flags will also bring up any boundary disputes you may have with your neighbors. If, for example, the town says that your piece of property ends at a certain point, but your neighbors think that they own a portion of the land you just bought, it’s better to clear up any disputes before you start building as they can take time and cause expensive delays.

A land surveyor will also place steel pins at all the corners of the foundation. “Pinning the foundation” is critical to making sure that prefab homes (or any other type of home for that matter) are meeting the “setback” requirements. Remember that modular homes are subject to all the same building codes as a site built home so they have to be “set back” from the property line by the required amount. After the foundation is laid, the land surveyor will come back, confirm that the foundation is actually the same as the pins, and then issue an “As-Built” survey for proof of compliance. 

3. Clearing and Leveling the Land

Next, you’ll need to clear and level the land so that the foundation can be dug. Depending on your skill and the type of clearing that is being done, you may be able to do some of this work yourself to help offset some labor costs. But, most of the work involved will need to be done by professionals with heavy duty excavating equipment. Clearing the land includes cutting down trees, not only on the build site, but also removing any unhealthy or dead trees that are close enough to the house to do damage if they fall. Tree stumps and large rocks will also need to be removed for the ground to be properly leveled. Remember that you also have to clear a path to the foundation that is large enough for your modular home to be delivered and have a space cleared that is large enough for the crane to set the actual prefab home units. This is where modular home builders, such as Next Modular, can be very helpful with providing the necessary information to plan accordingly.

With those steps completed, you’ll be able to move on to installing your foundation. Now if you’ve spoken to any modular home builders in the past you know that you can not build your new prefab home on a slab foundation. Your modular home builder will need access to the underside of the home to connect the mechanicals such as electric, gas, water and sewer. With that said you will need to select either a crawl space foundation or a basement foundation. If you’re still getting used to the terminology, here’s a brief description of both.

Crawl Space Foundation:

As the name suggests, crawl space foundations create a 48 inch deep crawl space within the foundation of the home. It features poured concrete or cinder block walls as well as concrete footers that go into the soil. This provides support for the weight of modular homes. When potential groundwater infiltration is a concern, a 6″ layer of pea stone is placed over the bottom of the crawlspace floor, and a sump pump is installed in the sump pit to eject any water that makes its way under the home. A visqueen (plastic sheeting) moisture/vapor barrier is always installed in a crawlspace in order to control moisture coming up through the ground. Sometimes a 2 to 3 inch concrete floor is poured over the top of the visqueen, but this is mainly for aesthetics or functionality (if you want to store things in the crawlspace), as the concrete will not stop water from coming in.

Basement Foundation

Basement foundations can create a reliable foundation for prefab homes as well as add additional living space. While basement foundations are one of the most expensive foundation options, a finished basement can significantly increase the square footage of your home – and, therefore, its value.

Once you’ve decided which type of foundation is best for your budget and needs, it’s time for installation. Pouring a concrete basement or crawl space involves a lot of specialized equipment so you will almost certainly need to hire a contractor to do this part for you. If you’re not sure who to hire, Next Modular can absolutely make some recommendations. Whether you’re installing a basement or crawl space, the process is basically the same. 

  1. Dig a hole at the appropriate depth for whichever foundation you require. What are we going to do with all that dirt? Normally the excavator will pile all of the dirt from the basement/crawl space hole together at the edge of your property. When the foundation is completed, they will use that dirt to backfill around the perimeter of the foundation and complete a rough grade, which will get the ground sloping away from the home according to required codes.
  2. With your hole dug, you’ll need “footers” installed before anything else. They are typically made of concrete with rebar reinforcement that has been poured into an excavated trench at the bottom of your foundation hole. The purpose of footings is to support the foundation and prevent settling. Because your prefab home will be delivered in pieces you’ll obviously need support posts under the seams so don’t forget to make sure that you have a footer under those posts as well. Next Modular typically does a “ribbon footer,” which is a long footer that runs the whole length of the marriage walls (where the sections of your home come together).
  3. Next the concrete contractor will set wall forms on top of the footers for your foundation walls, and install rebar to code. Once forms are up and ready, they will bring a concrete truck in to pour concrete into the forms. The forms can be vibrated to help the wet concrete consolidate and settle properly into the forms, in order to avoid unsightly air pockets.
  4. After the walls have cured and the wall forms are removed, the foundation will need to be sealed. You may not know this but concrete, though strong and tough, can have small pores or cracks that allow water and gasses to pass through it over time, so it’s important to apply a liquid sealant to the outside of the foundation walls before the dirt is backfilled. Foundations can be “damp proofed” or “waterproofed,” and which option you choose will largely be dependent on site conditions. 
  5. If you’re prepping for a future bathroom in a basement, now is the time the plumber will come in and install drain lines and an ejector pit in the dirt floor. Once this is done and inspected, the basement floor can now be poured. The floor, like any other concrete floor, will be smoothed out for a nice finish.
  6. Last but not least is installing sill plates for your new modular home to attach too. The sill plates will install with bolts or tie down straps that are wet-set in the concrete at the top of the wall. Your new modular home will rest on these sill plates and also be fastened to them. Now your foundation is prepped and ready for the modular home to be set!

Once you’ve installed the foundation for your new modular home, you’ll need to think about a few more details in order to be ready for your prefab home to be delivered. First and foremost you’ll need to think about drainage. Depending on the ground topography, soil conditions of your property, and/or your proximity to water such as a lake or river, you may want to consider taking additional steps to prevent water issues during times of heavy rain. 

There are four basic ways to manage water around your home, which we’ll explain in a minute. Your property may or may not need all of these, and modular home builders are also very helpful at giving you advice about what would be appropriate for your location, so don’t be afraid to ask questions. Managing water comes down to four basic methods:

1. Footing Drain

This is a highly effective water drainage system, and in fact it is required by code to have at least one footing drain around the exterior of every foundation (but you can do more if needed!). A perforated pipe is installed around the perimeter of the foundation walls on level with the footing. This pipe draws water away from the walls of the basement or crawlspace and drains it into the sump pit. The drain is covered with pea stone to a depth of 6 inches, and then backfill is placed on top of that, up to the desired final grade. If your lot is sloped enough, this footer drain can be run away from the foundation (instead of to the sump pit), and made to drain out onto the ground downhill from the foundation. We call this “draining to daylight,” and it is the best way to install a footer drain, because you will never have to worry about your sump pump going out and letting water fill up your foundation.

2. French Drain

This is a perforated pipe that is installed in a flat yard to drain away standing water. It involves excavating a trench that slopes away from the property and a pipe is then installed. A porous material such as gravel is used to fill the trench before the trench is finally covered. Standing water follows the easiest escape route, meaning it will seep down to the French tile and drain away from your yard.

3. House Gutters

Every home should have gutters, because water runoff from your roof can cause havoc if not dealt with. A gutter and downspout system help drain all roof water away from the house walls before it has the chance to seep down through the soil against the foundation wall and potentially in through the wall. The downspout drain should extend (above or below ground) far enough away from the house that the water can seep into the ground safely away from the home and not end up in your basement or crawlspace.

4. Proper Grading

This is a simple technique that involves reworking the grade so that it slopes away from your house. In fact, it is required by code to have at least 6 inches of fall within the first 10 feet away from the outside of the foundation (with some exceptions). It is also advisable to assess the way water will flow on and around your overall lot, because you may want to install swales (shallow “trenches” in the ground) to move water where you want it to go (which is AWAY from your modular home foundation).

Not all of these drainage solutions are directly part of installing a foundation but there are often overlapping considerations to be made with the excavating portion of the foundation installation process. Talk with your modular home builder to decide the necessity and best timing for these different drainage solutions. 

You Should Consider a Sump Pump for Your Home

Even with all these drainage solutions, there are still times when water can overwhelm your drainage systems and you need a sump pump. Sump pumps move water from your basement or crawl space and out of your home. A sump pump is located in a “sump pit,” usually a hole below the main surface of your basement floor. This pit, also known as a “basin,” holds the sump pump which is equipped with valves that sense escalating water levels or pressure. When the water gets too high, it automatically pumps excess water out of the basement and away from your property using a discharge line. Talk with your modular home builder to see if a sump pump is needed for your home. 

Account for Penetrations Through the Foundation for Mechanicals

The last thing you’ll need to consider when installing a foundation for your new prefab home is your mechanicals. There are a lot of different ways this part could go depending on the location of your property. If you are within city limits, you might have both water and sewer coming from city supply lines. If so you’ll need to consult with your contractor and the city about which side of your foundation is best to have your supply and drain lines stubbed out, which will depend on the location of the city’s water and sewer taps or stubs for your property.

Alternatively, you might have a permanent well for water with a leach field for sewer. Or possibly a mound system. You might have an electric-only house or electric with propane. Your modular home builder will be essential in helping you make the right decisions and preparations when both preparing your foundation and ordering your new modular home. For more information on options and costs for your mechanicals as well as many other parts of the prefab home building process, check out this article

At Next Modular, we know that choosing a home is likely one of the most important financial decisions you will make. As an experienced modular home builder, we are committed to being your guiding light through this process.

Call (574) 334-9590 for more information.


Next Modular builds modular homes in Indiana and Michigan. We typically take on Foundation & Set projects within a 250-mile radius of Goshen, Indiana, and full Turn-Key projects anywhere we have a local Project Manager. We can do “Home-Only” projects up to 500 miles out. Start your new home journey today by browsing one of our many modular homes for sale

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