Why the difference?
Often, with new construction, the tax appraisal reflects the land and the previous property that stood there. So, if it was a vacant lot (or, more likely, a lot with a tear-down on it) and the tax assessment is supposed to be at 100% of fair market value, the tax assessment is reflecting what was there prior to the new construction.
As for what you can offer the seller: You can offer anything you want. Recognize, though, as noted above, the $265,000 probably doesn't reflect the new construction. And, frankly, even if it did, never ever base an offer (or an assumption of fair market value) on a tax assessment.
Contact a Realtor and have him/her do a CMA on the property. It may be tricky--it often is when a neighborhood has multiple foreclosures or short sales--but it's much more reliable than relying on a tax assessment that may not even reflect current construction.
Hope that helps.