When you insure property, such as a home, you want to insure the property to its value. If the porch cost you $4,700 to build, you should insure it for at least $4,700. But, does the porch increase the value of the home by more than the $4,700 you paid for the porch? If your home was worth $150,000 before you added the porch, and the home is now worth $160,000 after the addition of the porch (just using random figures here), then you should increase your dwelling coverage by $10,000, instead of only $4,700.
If you have an agent, you might want to consult your agent. What type of homeowners policy do you have? Is it a replacement cost policy? Is it insured for the cost of the dwelling itself, or is it insured for the amount of your mortgage? And what exactly is the value of your home, minus the value of your land? These are a few questions you might want to make sure you know the answers to before you make a decision.
If you don't have an agent, you can usually call your company directly. But, know the answers to the above questions before you make a final decision.