Before we can discuss how much it will cost to build a foundation, you must decide on which type to construct. Are you putting a foundation under an existing building? That will require the additional expense of raising the house to put in a new foundation. Replacing a damaged foundation? Are you building a foundation for a new home or putting on an addition?
Whatever the objective for your foundation, you can spend thousands before you ever pour the concrete or put together concrete blocks to form a wall. Much of that expense will depend on how much excavation, tree removal, and leveling will need to be done.
Types of Foundations
A foundation could be as simple as a concrete slab, or as complex as a full basement. The more familiar you are with how foundations get constructed, the better. You’ll want to understand which types are best suited for your area’s soil, climate, and flood conditions. Of course, your budget could very well affect which foundation you choose as well.
Of course, everything depends not only on the type of foundation, but there are other considerations such as preparation, leveling, labor, etc. You must also add in rebar or wire mesh, sealing, draining, humidifying, insulation, and other necessities.
Most monolithic concrete slabs are four inches thick, poured into a form, with or without rebar or wire mesh, and often placed over packed gravel. These are primarily used in areas with moderate climates, though some use these for garages, sheds, and barns, so there is less concern with frost and thaws heaving the soil.
The next step up from a concrete slab is a stem wall. This foundation’s construction is similar to a crawl space. Thick forms get placed on top of footings where the outer walls will be, with anchor bolts set into the form. The concrete gets poured into the forms, and when the forms get removed, the sill plates are attached to the anchor bolts.
We have an abundance of information about calculating concrete slabs, by the inch, foot, yard, or even by the bag. Always round up to the next yard or add 10 percent overage.
The monopour concrete slab is usually the least expensive, depending on size and how much you can do yourself. Figure $3-4 per square foot.
Stem wall slabs are about $5.00 per square foot. However, when you add the prep and labor, you can usually add around $10 per square foot for each type of foundation.
Homeowners use a crawl space as a less expensive foundation to get the house off the ground and to protect it from flooding. Crawl spaces require vapor barriers, insulation, and possibly a dehumidifier to avoid mold growth. These are usually 18 inches above the ground but could be as much as three or four feet, depending on the flood danger and other necessities.
Crawl spaces average about $7.00 per square foot. However, insulation will add another $1-3 per square foot, but a vapor barrier is much cheaper at less than a dollar. You could install a dehumidifier cheaper, but you will pay for the electricity to run it.
Piers and Beams
Piers and beams foundations are very similar to a crawl space. However, these usually get used in rocky or ledged areas where you might not be able to dig footings around the entire building, or when there is a chance of soil shifting. You can use drilled piers or spread footings, and then stretch beams across the expanse. These can also get added to repair damaged foundations.
These run about $5.00 per square foot, and you can use the same figures for insulation, dehumidifier, and vapor barriers.
A full basement is, of course, the most expensive option for a foundation. However, there are many benefits. These can be fully finished or unfinished. They can be used for storage or even contain a garage. Some homeowners add a full basement or finish an existing basement as a method of adding living space, a mother-in-law suite, or a rental.
A full basement has many variables. A new basement will require excavation down to eight feet. It can cost anywhere from $10-20 per square foot, or $10,000 unfinished, once again depending on the size. For a finished basement, figure about $25-100 per square foot or well over $100,000.
The price will go up when you need a separate entrance. A finished basement will also require a quality drainage system, waterproofing, and sealing the concrete, which is a good idea anyway.
Footings serve as the base of the foundation and provide support for the structure. Learn more about the cost of footings that are necessary for the foundation.
Get a Minimum of Three Estimates
The strength and durability of your house or project depends primarily on the foundation. Use our calculators to help you develop a plan, so you will know what a reasonable estimate is. Just don’t make your plan so “concrete” that you won’t change it when contractors present a valid reason. You should also consider getting multiple estimates from licensed contractors before you start the project – most suggest at least three estimates are a good idea.