What exactly is a gravel pad?

A gravel pad is a compacted layer of crushed stone - typically 4 to 6 inches deep - laid over excavated and leveled soil. It acts as a stable, well-draining base for structures that sit on skids or blocking rather than being anchored directly to concrete.

The concept is simple, but execution matters. You need the right gravel (usually crushed stone, not pea gravel), proper edging to keep it contained, and solid compaction at every layer. If any of those steps are skipped, you end up with a pad that shifts over time and creates leveling headaches down the road. For tiny houses with a built-in wood floor, a properly built gravel pad handles the load just fine - the gravel can support any weight a wooden floor system can carry.

What exactly is a concrete slab?

A concrete slab for a tiny house is typically a 4 to 6-inch thick poured concrete base, reinforced with rebar or wire mesh, that sits on a compacted gravel subbase. It combines the foundation and the floor into one solid structure, which is why it's often called slab-on-grade.

The process involves soil compaction, a gravel drainage layer, a vapor barrier, and then the pour itself. Done right, a concrete slab can last close to 100 years with almost no maintenance. That's a real number, not a marketing language. But done wrong - on poorly compacted soil, without proper drainage - it cracks and settles, and repairs are expensive.

Gravel pad vs concrete slab: the key differences at a glance

When a gravel pad is the right call

A gravel pad works best when you want a solid foundation without locking yourself into one location permanently. It's the go-to option for most prefab and manufactured tiny homes, especially those delivered on skids and blocked up off the ground.

Here's where proper site prep makes all the difference. Companies that specialize in this kind of work - like the engineering-focused foundation company Site Prep - approach gravel pads as a real foundation project, not just a layer of rocks dumped on the ground. Soil is tested, the area is excavated to the right depth, compaction happens in layers, and edging is installed to keep the gravel from migrating outward over time. That level of attention is what separates a pad that stays level for 10+ years from one that starts shifting after the first hard winter.

Gravel pads are also the top drainage option. Water moves through stone and away from your structure, which keeps moisture from building up under the floor and causing rot or mold. This is especially important in wetter climates where concrete slabs can trap surface water if grading isn't done precisely.

Ideal conditions for a gravel pad

• Your tiny house has a built-in wood floor and sits on skids;
• You may want to relocate the structure at some point;
• Your site has good natural drainage or sandy/loamy soil;
• Budget is a priority and you want to keep costs lower upfront;
• You're in a mild climate without severe frost heave issues;
• Local zoning doesn't require a permanent foundation.

When a concrete slab makes more sense

A concrete slab is the right foundation when you're building a tiny house that you intend to be a legal, permanent dwelling. Most building departments require a poured foundation if you want the structure classified as a residence for zoning or mortgage purposes. If you're going for a building permit and want your tiny house to be on the books as a real home, a slab is typically required.

Cold climates are another situation where slabs have a clear advantage - but only if the slab is designed correctly for your frost depth. A slab poured too shallow in a northern state will heave and crack when the ground freezes. Done right, with proper insulation and edge treatment, a concrete slab actually performs better in freeze-thaw cycles than a gravel pad does.

If your tiny house has no built-in floor - meaning it's open underneath and the foundation also serves as the floor surface - a concrete slab is essentially your only real option. There's nothing else that can function as both structural support and a finished floor at the same time.

Ideal conditions for a concrete slab

• You want the tiny house classified as a permanent dwelling;
• Local code requires a poured foundation for permits;
• The structure has no built-in floor and needs a solid surface underneath;
• You're in a cold climate with deep frost lines;
• Long-term stability and near-zero maintenance are priorities;
• The site is flat and has well-compacted, stable soil.

What about the prep work - does it matter for both?

Absolutely, and this is where most DIYers cut corners and regret it. The quality of what goes under your foundation determines how well it performs for years. Whether you're building a gravel pad or pouring concrete, the steps before any material hits the ground matter just as much as the material itself.

For a gravel pad, that means excavating 6 to 8 inches of topsoil, compacting the subbase, and then adding and compacting crushed stone in layers - not all at once. For concrete, it means a similar excavation, a 4 to 6-inch gravel drainage layer, a vapor barrier, and steel reinforcement before the pour. Both foundations also benefit from being sized a few feet beyond the actual footprint of the house - the standard recommendation is at least 3 feet on each side - to prevent edge erosion and give you a clean perimeter to work with.

Skipping any of these steps doesn't just reduce the quality of the finished foundation - it creates problems that compound over years. A slightly unlevel pad gets worse, not better. A concrete slab poured on soft or wet soil will crack, sometimes within a single season.

Cost breakdown: what should you actually budget?

Gravel pads are significantly cheaper if you're doing the work yourself. At the time of writing, material costs alone can come out to around $2.40 per square foot if conditions are ideal. Hiring a contractor to install a gravel pad runs $5 to $9 per square foot depending on your region and the site conditions.

Concrete slabs for a tiny house footprint typically land between $4,000 and $12,000 fully installed. The wide range reflects variables like slab thickness, reinforcement type, local labor rates, and whether significant site grading is needed first. You'll also need to factor in permit fees, which for a concrete foundation can add several hundred to over a thousand dollars depending on your county.

In either case, the foundation is not the place to try to save money by cutting scope. A failed foundation means lifting the structure, removing and redoing the base, and potentially repairing damage to the building itself. The extra few hundred dollars spent on proper compaction and adequate depth almost always pays for itself within the first couple of winters.

Climate and soil: the two factors most people forget

Your local climate and your soil type should be part of this decision - not an afterthought. In areas with expansive clay soil, for example, both foundation types need special attention. Clay expands when wet and shrinks when dry, which creates movement that can undermine even a well-built pad or crack a slab that wasn't properly designed for it.

In northern states with frost depths of 36 inches or more, a gravel pad may shift noticeably over winter. That's manageable - you re-level the house in spring - but it's a real maintenance task, not just a theoretical inconvenience. A concrete slab in those same conditions, if it doesn't extend below the frost line, faces the same heaving problem. In the South and Southwest where winters are mild, both options perform reliably with minimal seasonal movement.

Soil drainage is also worth checking before you choose. Sandy or loamy soil drains well and is generally friendly to gravel pads. Heavy clay or areas with a high water table are trickier - a gravel pad in those conditions may still hold up, but you'll need to be especially thoughtful about site grading and potentially add a French drain around the perimeter.

Final call: which one should you choose?

For most tiny house owners - especially those placing a prefab or manufactured home on their property - a properly built gravel pad is the smarter, more practical choice. It costs less, drains better, doesn't require a concrete contractor or a permit in most jurisdictions, and still gives you a stable platform for decades if it's installed correctly.

Choose a concrete slab if you're building a permanent dwelling that needs to meet local building codes, if you're in a cold climate with a deep frost line, or if your structure has no built-in floor system. The slab is more expensive and more work upfront, but it's also the more permanent and code-compliant solution when those things matter.

Either way, the foundation is the one part of this project you don't want to treat as an afterthought. Whatever you build on top of it is only as solid as what's underneath.