MUMA’s “highly-original” extension and refurbishment of Manchester’s Whitworth art gallery has made the shortlist of six for this year's Stirling Prize.

It was described by organisers the Royal Institute of British Architects (RIBA) as “a project for all seasons, where art, nature and architecture combine” which “could be the eulogy for a building which is neither high-key nor overtly fashionable”.

The Whitworth reopened on 14 February following London-based MUMA’s £15m (US$23.4m, €21.5m) renovation, which doubled the public space and extended the exhibition floor, while incorporating educational areas and a storage centre.

The gallery won this year's £100,000 (US$156,200, €143,250) Art Fund prize for Museum of the Year on 1 July.

It joins a University of Greenwich building located in a UNESCO World Heritage Site, two housing projects, a school, and a cancer care centre on RIBA’s shortlist. The architects involved in these projects are Allford Hall Monaghan Morris, Niall McLaughlin, Reiach and Hall, Rogers Stirk Harbour + Partners and Heneghan Peng.

Recognising the UK's best new building, the Stirling Prize is now in its 20th year, with Haworth Tompkins’ Liverpool Everyman Theatre picking up the award in 2014.

The 2015 winner will be announced in London on 15 October.