The small town of Plan-les-Ouates, in the suburb of Geneva, built a 10'000 m2 multifamiliary building, including some commercial and administrative areas. It is situated in the centre of the town, with relatively high density constructions. The owner wanted urban buildings with a very low energy consumption, but with moderate extra cost, becoming good examples of “the state of the art” in this field. The envelope of the building is very well insulated and the solar gains are optimised, reaching a calculated heat demand close to 100MJ7m2 year. Air heat recovery systems (buried pipe + exhaust air heat exchanger) and 1400 m2 of uncovered solar collectors coupled with a 100 m3 storage allow to decrease the gas consumption. This low cost solar roof is the greatest in Switzerland and, maybe, in Europe. An extended two years monitoring has been in progress since spring 1997 in order to evaluate the total energy consumption of the building and to study the different subsystems (envelope, heat recovery systems, solar collector, gas boiler). Special attention is given to the solar roof, its architectural integration and its thermal performance. Application of this low cost solar roof technology for the South of Europe will also be studied, specially for Portugal. The building's features and the first results will be presented in this paper.