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No, it's not Florida. Still, the wall-to-wall sand, open-air benches, palm tree and gambolling toddlers are all real enough, even if sunshine is a less than dependable fixture in Northamptonshire.

This is Greenhill Jenner Architects' extension to Pen Green Centre in Corby. Far more than a nursery school, Pen Green is a trailblazing project in the government's Sure Start initiative, which sets out to give an all-round boost to young children in disadvantaged communities.

Since the closure of its steel mill, Corby fits the description of a disadvantaged community. The Pen Green Centre attempts to compensate for this with an assortment of education, health and social facilities – not just for the children but for their families, too. As well as conventional nursery classrooms, it contains rooms devoted to water therapy, multisensory relaxation and strenuous activity for children, along with training and counselling rooms for parents and would-be parents.

The design influences behind Greenhill Jenner's extension to the 1930s school building are actually more Italian than southern American, as they are based on advanced theories of nursery design developed in Reggio, Italy. The two centrepieces are a child-sized piazza and a three-storey campanile overlooking it at one corner. In the words of director John Jenner: "The piazza is a meeting place for the community, and the campanile gives a point of reference for the whole centre."

In reality, the piazza is an extended children's sandpit, and the campanile is nothing more than a stair tower wrapped around a large high-level water tank. "The centre needed a large cold water supply," explains Jenner. "So we had the idea of storing the water at high level and feeding it by gravity, rather than installing an expensive pumping system. Then we built the stairs around it with small viewing windows and a weather-station at the top for the children. We turned a plumbing facility into a play feature."

In the extension, the facilities overlook the piazza through window walls on three sides. On the fourth side, the main nursery room in the old building has been opened up to the sandpit and the sky by replacing a solid brick wall with a series of French windows. A timber deck, bench and projecting roof canopy surround the sandy piazza and integrate new and old buildings.