Cost approach may be used to show a "value in use", but it doesn't represent the market or sales approach value. Cost approach is also usually thrown out in the case of older structures, as the cost to rebuild doesn't pencil out when you figure in the accumulated depreciation and remaining effective useful life of the improvements.

Two homes, one 5 years old, and one built brand new, same amenities and size, would likely appraise for the same value, regardless of the cost of construction. TAV might be different, but at the moment there is very little entrepreneurial profit benefit when building a new home. Your better value right now is to buy an existing home, with very few exceptions. Building your own home will likely cost you more right now than youcan buy the same home for, especially if you contract it all out.

(Sorry, I do appraisal reviews for my bank)

DBappraiser is also spot on when he says that the appraiser doesn't determine the selling price for a home. In an arms length transaction with a willing buyer and seller, they are the ones who determine the selling price of the property. The job of the appraiser is to determine the value of the home based on comp sales and directions of their client.