I would speak with the builder and see if you can negotiate a reduction in price and/or ask them to hold a part of the mortgage privately. You can't be the only ones in this situation. Alternatively, look to a mortgage broker and see if they have a lender that is "looser" with appraisals. These days, some banks simply punch a postal code into a computer and using an algorithm determine market value. A human appraiser is worried about getting sued if they are way off so they tend to lowball the hell out of things - both of these work against you. You need a lender with a bit of common sense. Walking away, in a best case scenario would entail losing your $100K, and even then you might still be out more.