Foundation choices
Slab-on-grade vs foundation and crawlspace or basement
...slab floor
Follow the advice in THE ECOHOME A TO Z SLAB BUILDING GUIDE
Further reading for Slab on Grade or basement construction:
How to choose between a crawlspace or a slab foundation ? - Ecohome Guide
Best practices for basement construction - Ecohome Guide
How much insulation does a basement need - EcoHome Guide...