Median sales prices can rise because of increases in fair market value – i.e. buyers are paying more money for the same home (supply and demand) – and/or because buyers are purchasing larger, more expensive houses. Both dynamics are currently at play, but it is certain that affluent and very affluent buyers have made up a dramatically increasing percentage of the total market throughout the Bay Area. In San Mateo County, the average size of houses sold in Q3 jumped a whopping 11% year over year, which explains some of the enormous increase in median house sales price.