lightbrite,
Does someone who is frum and knows that a double sink and kasherable one makes life easier have a responsibility of making a kosher-friendly space?
Or can one renovate based on interior design trends, hoping to get more bang for the buck regardless of how practical it is for a person who is frum to live.
How likely is it that the subsequent purchasers would be Jewish? If the house you are flipping is in a Jewish neighborhood, I would think that a kosher-friendly kitchen would be a pretty big selling point.