One of four disused gasholders in King's Cross, London, is set to undergo a major £2.5m makeover after designs drawn up by architects Bell Phillips + Kimble were selected for the scheme.

The London-based practice was selected from a shortlist of five firms following the King's Cross Central Gasholder No. 8 design competition, which aims to transform the Grade II-listed structure situated to the south of the Regent's Canal. Bell Phillips + Kimble's designs for the 150-year old gasholder feature a new water pool and garden, flexible internal spaces and a natural amphitheatre for informal performances and relaxation.

The large shallow pool will be used as a children's play space during the day and as a public space at night, while beneath the 'roof-top' pool will be an events space with glass lenses in the ceiling offering views of the gasholder structure. Bridget Evans, project director at King's Cross Central, said: "We asked for big ideas and that is certainly what we got. The winning design works on several levels for King's Cross Central - it will be accessible with multiple uses, it's fun, has style and it will create an enchanting space for local communities to enjoy."

Hari Phillips from Bell Phillips + Kimble added: "The combination of the gasholder structure and its location overlooking the Regent's Canal sets the scene for a truly fantastic project." Other shortlisted designs included a helter skelter slide drawn up by Feix and Merlin; landscaped space put forward by Gustafson Porter; Hakes Associates' plans for an events space within a reflective ball and Loop Architecture's pavilion and landscaped area proposals.

Work is expected to get underway on transforming the 25m (82ft)-high gasholder in 2011, subject to receiving planning permission and listed building consent. The removal, refurbishment and reconstruction of the gasholder will not be included in the £2.5m construction budget.