The Winter Garden is the largest city centre botanical garden in Britain and the associated galleries are among the most flexible venues for travelling exhibitions outside London. The two parts of the project are strikingly different. The huge glasshouse has a powerful, almost animal quality - a giant rib-cage - and yet makes direct reference to trees in the massive glue-laminated parabolic arches. The heavy aroma of damp earth and lush vegetation is almost shocking in the noisy, hard-edged, fumed-filled centre of Sheffield.

By contrast the Galleries are cool, calm and precise, a carefully controlled essay in heavy-weight prefabrication 'The Avenue' is a sequence of tall generous spaces with a remarkably light, relaxed quality that runs the full length of the building, serving as a public foyer, an orientation space for the Galleries and a link between the Winter Garden and Sheffield Hallam University to the east. In the Galleries themselves the architects have achieved a difficult balance between spaces of strong architectural character and a very high level of adaptability. These rooms could have been rather dull – constant height, insistent grid to accommodate variable display layouts and bland will-go-with-anything colours, but controlled top-lighting, high quality materials and restrained but robust detailing combine to create a flexible space with a sense of occasion. It may prove a difficult space for curators but the flexibility does offer the possibility of a wide variety of artistic experience.

But the real strength of these buildings is as urban design. There is an intriguing ambiguity in their spatial qualities – are they indoors or outdoors, routes or destinations? It is very much a case of both-and rather than either-or. As the architects note, these are not so much ‘objects in an open city square, but streets and spaces mined out of a dense bit of urban fabric …new routes that lead to other destinations, linking three city squares’.

Together these two strikingly different but remarkably complementary interventions are a delightful, surprising and adroit addition to the fabric of a great city.