Housing affordability problems have become more serious over the course of the last few decades and are now also affecting the middle-class, despite the fall in prices in the housing market. This study proposes a methodology to assess threshold income as an index for measuring housing affordability by applying a combination of the ratio income and residual income approaches. The methodology is applied to two particular areas of Sicily in Italy as case studies consisting of medium-size metropolitan areas located in a less developed European region. The areas have been chosen on the basis of their different territorial structure: a polarized area that comprises a high-density city center and a polynuclear urban region.