Planning a garage, workshop or addition this year? Before anyone talks square footage or door styles, there’s a decision hiding under the whole project: what does this building stand on? In New Hampshire, that question has three main answers — a monolithic slab (you’ll also hear it called a floating slab or an “Alaskan slab”), a frost wall with a slab, or a full foundation — and our winters make the choice matter far more here than it does in most of the country.
Here’s what each option actually is, how they compare, and how we help customers choose the right one for their lot and their plans.
The three options, defined
Monolithic slab (floating slab / “Alaskan slab”)
This option goes by several names, and they all describe the same family. A monolithic slab is one continuous pour — slab and thickened edges placed together over a properly prepared and drained gravel base. “Floating slab” describes how it behaves: the whole structure floats on the ground rather than anchoring below frost depth, relying on base preparation and drainage to handle seasonal movement. And “Alaskan slab” is the New England nickname for the cold-climate version — a monolithic pour with a thickened, often insulated, perimeter edge designed with frost in mind. Whatever you call it, it’s the simplest and most economical of the three.
Frost wall with slab
Concrete frost walls on footings that extend below the frost line — commonly about four feet down in most of New Hampshire — with a slab poured inside them. The building is anchored in ground that never freezes, which is the code-standard way to protect attached and heated structures from frost movement.
Full foundation
Full-height walls creating a basement or tall crawl space beneath the building. Everything a frost wall offers, plus usable space below — at full-basement cost.
How they compare
| Monolithic slab | Frost wall + slab | Full foundation | |
| Relative cost | $ | $$ | $$$ |
| Frost protection | Base prep & drainage | Anchored below frost | Anchored below frost |
| Best for | Detached, unheated | Attached, heated, garages | Living space, storage below |
| Adds space below | No | No | Yes |
| Typical timeline | Days | 1–2 weeks | 2–4 weeks |
Why New Hampshire frost changes the answer
Frost heave — the ground physically lifting as water in the soil freezes and expands — is the force this whole decision manages. A structure founded above frost depth can be lifted unevenly each winter, and uneven movement is what cracks slabs, racks door frames and opens gaps. New Hampshire’s frost commonly reaches about four feet down, among the deepest in the country, which is exactly why our building code requires permanent structures to be frost-protected — by frost walls and footings below the frost line, or an approved frost-protected design.
Two factors do most of the deciding. Attached or detached: anything attached to your house must move exactly as your house does — which means frost-protected, full stop. Codes allow more flexibility for smaller free-standing accessory buildings, which is where monolithic slabs earn their keep. Heated or unheated: a heated building keeps the ground under it warmer and shifts the frost dynamics at its edges; heated space generally deserves — and is generally required to have — frost-protected support.
Choosing by scenario
Detached storage building or shed
A well-built monolithic slab over a properly compacted, well-drained base is usually the right call — most of its long-term performance is determined by the excavation and base work you never see.
Detached garage
This is the genuine judgment call. An Alaskan-style monolithic slab can serve a detached, unheated garage well on a well-drained site; a frost wall buys permanence, easier future heating, and resale confidence. Soil, drainage and your ten-year plans for the building tip the decision — this is exactly what a site walk is for.
Attached garage or addition
Frost-protected — a frost wall at minimum. Attaching a floating structure to a frost-anchored house builds a differential-movement problem directly into your home.
Workshop, or anything you might convert to living space
Build the foundation for the building’s future, not its first year. Upgrading a foundation under an existing building later is among the most expensive retrofits in construction; a frost wall (or full foundation) now is dramatically cheaper than regret.
What your building department will say
Foundation requirements come from the state building code as administered by your town — and approvals genuinely vary from one town to the next, especially the exemptions available for small accessory structures and the details each building department wants to see. Before finalizing plans, it’s worth a conversation with your town’s building inspector.
How we help you decide
We walk the site, look at soils, slope and drainage, ask what the building needs to do now and might do later, and quote the sensible options side by side — including the excavation and base work that concrete-only bids leave out. Sometimes the honest recommendation is the cheaper one; a monolithic slab done right beats a frost wall done wrong every time.
Planning a garage or addition this season? Call N.C. Bratko Construction at (866) 927-2856 for a free site evaluation and an itemized quote on the options that fit your lot.
Frequently asked questions
Do foundation requirements vary from town to town in New Hampshire?
Yes — more than most people expect. New Hampshire’s building code is enforced locally, towns can adopt their own amendments, and the exemptions for small accessory buildings in particular differ from one building department to the next. Check with your town’s building inspector before finalizing plans.
What’s the difference between a monolithic slab, a floating slab and an Alaskan slab?
Mostly vocabulary. All three describe a slab and its thickened edges poured as one piece over a prepared gravel base. “Monolithic” describes the single pour, “floating” describes how it rides on the ground instead of anchoring below frost, and “Alaskan slab” is the common name for the cold-climate version with a thickened, insulated perimeter. Ask three contractors and you may hear all three names for the same product — what matters is the base preparation and edge detail, not the label.
Can a slab garage be converted to heated space later?
Sometimes, but it’s the expensive path — heating a monolithic-slab building raises insulation and frost questions that are cheap to solve before the pour and costly after. If heat is even a maybe, say so at the design stage.
Is a monolithic slab okay for a two-car garage?
For a detached, unheated two-car garage on suitable, well-drained soil — frequently yes, and it’s a popular choice for good reason. The base preparation is everything; that’s where corners get cut and slabs get cracked. And because town requirements differ, confirm with your building inspector that a slab is approvable for your project before you fall in love with the price.
What actually fails when a foundation is under-built?
Seasonal movement: cracks that open and close with the calendar, doors that stick every February, and, on attached structures, gaps opening at the connection to the house. It’s rarely catastrophic and always chronic — which is why it’s worth preventing for the life of the building.
About the author: Nathan Bratko is the owner of N.C. Bratko Construction LLC, a full-service concrete, excavation and septic contractor based in Hillsboro, NH, serving Concord, Hooksett and communities across central and southern New Hampshire for more than 20 years. Free estimates: (866) 927-2856.
