I don't think that the square footage has affected the value of your home. When a property sells the appraiser has to use comparable sales and listing to come up with the value. Square footage is one of the main factors that is used. Adjustments can be made for square footage. If the add ons where made to the home without building permits then the auditors site has no record of them. the main thing that has affected a drop in price is the amount of foreclosures in the area. If a property sells under market value and is in the neighborhood and is used as a comparable it is going to bring your property value down.