Generally, you would want to purchase hazard insurance (with replacement cost coverage) for the value of the structure. If you only insured your balance of lets say $20,000, and the value of the home was $100,000, the home would be seriously underinsured, and the insurance company may ony pay 20% of damages in the case of a loss, in this example due to only it only being covered for 20,000 or 20%, the actual policy probably describes what are the consequencies if the structure is under insured. So finding out from the insurer up front how coverages are affected if underinsured would be a good idea.