There are three concrete foundation types you will actually build on in Eastern Idaho: a full basement, a crawlspace, or a slab-on-grade. Around here the full basement wins most of the time, and the reason is frost, not preference.
I run Skinner Customs out of Rigby, and I pour all three. What follows is how the choice actually gets made on a real lot, not a textbook list.
Frost depth already decided part of this for you
In Bonneville County the minimum footing depth is 36 inches below finished grade, and 42 to 48 inches is common once an attached garage slab is in the picture. Jefferson, Madison, and the neighboring counties land in a similar range, but confirm the number with that building department rather than assuming.
Read that again and the basement math gets obvious. The excavator is already going down three to four feet for the footing on any foundation type. Going down another four or five feet and pouring taller walls is the cheapest square footage you will ever add to a house in this valley. In a climate where the hole has to be shallow, slabs win. In ours, they usually do not.
That is why the standard custom home in Rigby, Idaho Falls, Rexburg, and Ammon sits on a full basement, and why the slab homes you see here tend to be single-story builds, shops, and additions.
The three types, straight
Full basement. Footings below frost, poured concrete stem walls eight to nine feet tall, a slab floor inside. Gives you mechanical space, storm shelter, and finishable square footage. It is the standard for most custom homes in the valley at $22 to $40 per square foot of basement floor, unfinished. A walkout on a sloped lot runs $28 to $50 because the footings step, the forming gets more complicated, and the waterproofing detail matters more.
Crawlspace. Same footing depth, shorter stem walls, no floor slab, just a vapor barrier over the dirt. Lands between the slab and the basement depending on wall height and how much of the site has to be dug. It buys access to plumbing and ductwork without the cost of a full basement. It also gives you a space that has to stay dry and vented correctly for the life of the house, which is where most crawlspace regret comes from.
Monolithic slab-on-grade. Footing and floor poured as one piece, thickened at the edges and under bearing walls. $8 to $12 per square foot. Least expensive, least flexible. Works on stable ground for single-story homes, and it is the right answer for most shops and outbuildings. Everything mechanical is either in the slab or in the attic from then on, so a plumbing change later means saw-cutting your floor.
What the ground gets a vote on
The dirt decides more than the floor plan does.
Water table and irrigation history. A lot of ground around here is old farm ground with canals, laterals, and a spring water table that sits higher than people expect. A basement on that lot is not automatically off the table, but it needs real drain tile at the footing, a sump, and honest grading. If nobody has looked at seasonal high water on the lot, that gets checked before the type is picked.
Soil type. Silt, clay, and uncompacted fill all behave differently under load. The residential concrete code, ACI 332-20, covers footings, foundation walls, and slabs-on-ground, and carries an entire separate chapter for design on expansive soils. That chapter exists because the wrong assumption about dirt cracks foundations. On questionable ground you want a soils report, not a guess from the guy bidding the concrete.
Slope. A lot with fall on it wants a walkout or a daylight basement. Flat ground makes a slab or a crawlspace easier. Fighting the lot costs money in both directions.
What does not change with the type
Whatever you pour, these do not move:
- 4,000 PSI minimum, air entrained. Freeze-thaw here is not a design detail, it is the whole climate.
- Rebar, not wire mesh. #4 on an 18-inch grid, chaired up so it sits where it belongs. Mesh is the low-bid shortcut. It gets walked to the bottom of the pour and does nothing.
- Compacted gravel base, minimum 4 inches, in lifts. Deeper on soft or filled ground. Concrete does not span voids.
- Footings below the county frost depth. Confirmed off the approved plans, not off the last job.
- Drain tile at the footing and positive grade away from the building. Frost needs water. So does a wet basement.
- Backfill after the floor and the framing are tying the walls together, not the afternoon the forms come off.
A cheap bid usually comes from moving one of those six, and five of the six are buried before drywall.
Where the money actually shifts
People assume the foundation choice is a line item they can shop. It is a structural decision with a tail on it.
A basement adds cost at the pour and gives it back as finishable space. A slab saves at the pour and takes it back the first time a plumbing line has to change. A crawlspace sits in the middle and depends almost entirely on whether the ventilation and vapor barrier were done right.
The expensive version of all three is the one that gets changed after the plans are stamped. Stepped footings, garage elevation, window wells, and stair locations are all cheap on paper and painful in the dirt.
Pick it before the plans are final
The order that saves money: soils and water table first, then foundation type, then the floor plan drawn to fit it. The order I see most often is the reverse, and it is why a dig stops while somebody calls the county.
If the concrete contractor sees the plans before they are stamped, you get a tighter bid and fewer change orders. That costs nothing to do.
Frequently asked questions
What are the three main concrete foundation types?
Full basement, crawlspace, and slab-on-grade. In Eastern Idaho the full basement is the most common choice for a custom home because the footing already has to go below frost depth.
Is a basement worth it in Idaho?
Usually. The excavation and footing depth are largely required anyway, and the added wall height buys conditioned square footage at a lower cost per foot than building up.
Can you build a slab-on-grade house in Eastern Idaho?
Yes, on stable ground, with the footing thickened and taken below the county frost depth. It is most common on single-story homes, shops, and additions.
What is the cheapest foundation type?
Monolithic slab-on-grade, at $8 to $12 per square foot. Cheapest at the pour is not the same as cheapest over thirty years.
Does a crawlspace need a concrete floor?
Not structurally. It needs a sealed vapor barrier over the dirt and correct ventilation or conditioning. Some owners pour a thin rat slab for cleanliness and access, and that is a preference, not a requirement.
Who decides the footing depth?
Your county building department sets the minimum. The approved plans carry it, and the contractor digs to the plan.
Get a straight answer on your lot
Send me the lot address and what you are planning to build. I will tell you what the county requires there, what the ground looks like for a basement, and what the foundation realistically costs for each type.
Need concrete work in Eastern Idaho? Call Zach at Skinner Customs at (208) 390-5794 or start at skinner-customs.com.
Content produced in partnership with S4 Media, marketing partner of SwagerBuilds LLC.
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