Several researches show the environmental and microclimatic benefits of the integration of vegetation in architecture; however the potentialities of vertical and horizontal greening systems to retrofit buildings are still not much investigated. The retrofitting project of the Barsanti Institute of Camogli (Genoa, Italy) is presented, a building dated back to the sixties with serious architectural and efficiency problems, located in a considerable landscape area. The development and application of a design tool (process tree), for horizontal and vertical greened surfaces, allows to evaluate the potentialities of vegetation to retrofit and to relate the encountered efficiency problems and the climate characteristics with the choice of plant species, system, and technology more suitable for the specific situation (of which environmental and economic impact are also evaluated) and to define a design approach for the systematic consideration of the many parameters involved.