Options:

• Concrete Basement
  • + More storage space
  • + Separate utility room
  • + More flexible option if site doesn't allow Slab on grade
  • - Higher carbon footprint (75 - 100 tons of concrete, 1 ton of greenhouse gas per ton)
  • - Higher cost ($20k - $40k)
• Slab on Grade
  • + Cheaper, less concrete than a basement (~$8/sq + ~1k-2k for finishing)
  • + Cheaper to lay plumping, wiring, etc.
  • + Provides thermal mass to absorb heat
  • - Requires a utility room on the main floor (should be sound-proofed)
  • - Harder floor on main floor

Conclusion: Go with Slab on Grade if the building site allows it. Depends on municipal restrictions for footprint/height, if building on a slope might not be possible, site drainage, also how deep the frostline is (foundation needs to be below the frostline). Regardless of type should insulate the surface extending 4 feet out from the home. Also lay radiant heat tubes. One of the most important points to keep in mind is that the correct foundation depends heavily on the site, e.g. soil type, drainage, slope, etc.

Main environmental impact is in the production of Portland cement (1 ton of carbon dioxide per 1 ton of cement). Can look into mixing recycled materials into the cement and/or capturing carbon inside the cement.

Ballpark estimate: $11,000. ($8/sqft * 1000sqft = $8,400 + $2,000 finishing)

References:

• Ecohome guide
• This Old House
• Strawbale Construction