Even when comparing exact model matches within a tract of homes, you should note whether the previous owners have made any substantial improvements. Cosmetic changes should be largely ignored, but major improvements should be taken into account. Most important would be room additions, especially bedrooms and bathrooms. Other items, like expensive floor tile or swimming pools should be taken into account, too, but should be discounted. A pool that costs $20,000 to install does not normally add $20,000 in value to the home. Rely on your agent to give you guidance in this area.