Christian - You may be able to negotiate a little site investigation before you actually purchase the property. The seller may want a geo-technical report anyway if your deal falls through. Speak to the contractor's in-house architect and ask them what they need to see in the geo tech report in order to feel confident designing the house there. Then, as a condition of closing escrow, put in a clause that the deal depends on the findings in the geo tech report. It sounds like you have everyone you may need to get going.
Josh, thank you for the insight. I appreciate your help with a more clear understanding of what to ask the contractor.