Your construction cost may or may not reflect the current market value of your property. It is only one element that is considered. Assessments are based on fair market value of the property not on the cost of the construction whether you did the work yourself or hired a contractor. Most residential improvements are based on what is known as contributory value. This contributory value is not based on cost, but rather the added value it "contributes" to the overall worth of the property. Cost estimates from three builders may range from $75-$85 per square foot. IF you decided to build or subcontract the construction project yourself, your construction cost may be between $65 to $70 per square foot. The Assessor must provide an opinion of market value.