The full basement may cost 3X what the corresponding slab size may cost from a concrete perspective. But you have to also consider the cost per square foot to build the structure that covers the slab/basement. If you double your slab size you need twice as large of structure to cover it. On the other hand the structure footprint is the same size weather it has a basement or a slab under it.

The full basement may cost 3 times what the corresponding slab size may cost from a concrete perspective. But you have to also consider the cost per square foot to build the structure that covers the slab/basement. If you double your slab size you need twice as large of structure to cover it. On the other hand the structure footprint is the same size whether it has a basement or a slab under it.