The foundation is decided by the ground, not by the house. Get a soil answer before you get a quote, because the three options differ by a factor of five in cost.
Screw piles are the default, not the answer
Helical piles are quick, reversible and need no concrete cure, which is why almost every tiny house supplier proposes them first. In firm sand or gravel they are a day's work for a two-person crew and they can be unwound if the house moves on. That reversibility is worth real money when the plot is leased.
They are not universal. In soft clay, peat or made ground the pile has to reach a bearing layer that may be four metres down, and at that depth the price stops being a fraction of a slab. Ask for a probe before you accept the quote, not after the rig arrives.
A strip footing suits a permanent site
Where the house is staying and the ground is uneven, a concrete strip under the two long chassis rails spreads the load and gives a level datum that does not need shimming later. It also raises the timber clear of splash-back, which is the single most common cause of rot at the bottom board.
The cost is groundwork and a cure period, so it lengthens the programme by a fortnight. On a plot where the crane and the excavator are coming anyway, that overlap is cheaper than it looks on paper.
A slab is rarely justified
A full slab is the most expensive option and buys you almost nothing a tiny house needs: it does not improve insulation, because the floor cassette is already insulated, and it removes the ventilated gap beneath the house that keeps the underside dry.
Where it does make sense is a permanent multi-unit site with services routed under the deck, or ground so poor that a raft is the only economical way to spread the load. Both are decisions for an engineer, not a brochure.
Levelling and access decide the day
Whatever the foundation, the house arrives on a lorry and leaves it on a crane. The crane needs firm standing within reach of the final position, and on soft ground that means mats. A foundation that is perfect and unreachable costs a second crane visit.
Set the levels before delivery day and check them with an instrument, not a spirit level across a plank. A house that is 15mm out over eight metres will show at every door.
Frost, drainage and the gap underneath
Every foundation type has to get below the local frost line or be designed to tolerate movement. That depth varies across the five markets and is the first number to ask the local authority for.
Keep the ground under the house draining away and keep the gap ventilated. A skirted, sealed underside traps moisture against the chassis and turns a fifty-year steel frame into a twenty-year one.