Why do you say building on a slab is more ecological than starting with a basement?
...on raises pertinent points. Basements are certainly useful, but from an environmental point of view, the benefits of slab-on-grade construction over a basement are undeniable, you can read all the pros and cons here -
Slab on Grade or Foundation and Basement; Which is Best?
And have a look at the Edelweiss House that ...