The Museum of San Telmo, in its present condition, represents the result of a long process of successive modifications which has partially altered its physical and functional character over the years. Its location on the fringe where the urban structure meets the topography of Monte Urgull is a reflection, on the other hand, of an urban problem very characteristic of San Sebastian: the solution of a division never completely solved between natural and artificial landscape.How to approach a contemporary extension of San Telmo in response to new requirements for space and stringent landscaping conditions, while expressing its connection to the location with the passing of time?The direct and radical gesture which defines out proposal implies paradoxically its practical dissolution in the landscape of Monte Urgull. We will limit ourselves to building a new green wall, deep and light, which is defined by the existing topography, and which hides in its interior two pavilions which will house the new programme. A “green wall”: on certain occasions the metaphor associated with an architectural idea gives a sense to each and every aspect of the project. Hence the slight changes of direction of the wall are sufficient to provide a natural solution to pedestrian access to Monte Urgull, to configure an open air exhibition space, or to house a café-terrace open to the landscape and to the town.The new extension of the San Telmo Museum will modify its appearance with the passing of the seasons: it will fade on occasions and blend with the vegetation on the hill, and will reappear on other occasions evoking a long unfinished wall: an unexpected metaphor – perhaps – of the difficult relation which architecture establishes with the pass of time.