Concrete footings and slab floors
No compromises: a good foundation is critical. Your house will only last as long as the foundations last. It is far better to err on the side of making the foundation deeper, wider and thicker than you think will be necessary.
However, all foundations are very site specific, and we always have an engineer design and certify our foundations, as we are not experts on soil types and their load-bearing capacities. It is also a good idea, before you buy your land, to research how foundations are commonly built in the area that you are considering building in. This way you will know what you are in for as far as foundations go.
In a strawbale structure, a good foundation is essentially a perimeter footing below the bale walls which is strong enough to support the entire structure, and high enough to raise the bales off the ground at least 150 mm, with 250 mm being the ideal height. The footing should be the width of the bales (450 mm is normal), and the surrounding land should be landscaped to slope away from the foundations to carry away any rain runoff. Though not individually heavy, a whole wall of bales weighs considerably more than a similar length of timber-frame wall, and nearly as much as a brick veneer wall. So, we tend to allow the footings to be at least the equivalent of the footings of a brick/masonry veneer wall (but wider). It is cheaper to use extra concrete at construction time compared to the expense of repairing cracking plastered walls or underpinning a settling foundation.
Consider what type of floor you desire in the building. If you are thinking of a concrete slab as opposed to an independent floor, then you will need to make the slab surface at least 50 mm below the level of the inside of the footings, where the bales will be sitting. It is a good idea to do this for the internal walls as well. This will allow any water to drain away from the walls during the construction stage before rendering, when the bales are vulnerable to moisture damage.