Mass appraisal is a group valuation techniques, that replicate the property market behaviour through a representative model. In some property markets there are few market data and the information sources are uncertainty, therefore econometric modelling may be difficulty or unreliability. In these cases econometric modelling of relationship between the dependent variable (property price) and the independent variables (property characteristics), may be difficult. Rough Set Theory is a property valuation methodology recently applied to property market (d'Amato, 2002). The model permit to estimate a property without defining an econometric model. In this methodology the econometric relation is replaced by an if then rule. The RST has been improved with a ëvalue tolerance relation', in order to make more flexible the relationship between rules and sample of observation (d'Amato, 2004). Both methodology, Multiple Regression Analysis and Rough Set Theory, has been tested on sample of residential property transaction. The data comes from the Real Estate Market Observatory of the University Polytechnic of Bari.