Manchester’s The Whitworth Gallery’s hashtag is #galleryinthepark. And when it re-opens to the public on February 14 visitors may well find themselves gazing out at the plane and ash trees of Whitworth Park, revealed and framed by McInnes Usher McKnight Architects (MUMA) glass, steel and brick extension, as much as at the Bacon, Freud and Paolozzi artworks inside.

Dr. Maria Balshaw, director of the Whitworth which is part of the University of Manchester, is confident that gallery regulars, will enjoy the “shock of the view”; park vistas now visible through glazed apertures that offer a new aspect where once were blank white walls.

Balshaw's brief to MUMA included a quote from one of her fellow director predecessors, Margaret Pilkington, who stated in 1932: “I have come to the conclusion that a good museum or gallery should be a place where people feel comfortable. If it stands in a garden or park, the visitors should be able to enjoy the beauty of the outdoors as a counterpoint to what is within.”

This egalitarian spirit informs The Whitworth's and MUMA's approach to the redevelopment (funded by the Heritage Lottery Fund, University of Manchester and Arts Council England amongst others) with a move towards a new transparency both architecturally and psychologically. Prior to the renovation the gallery canvassed local opinion about the existing building, originally built in 1889 and a combination of austere red brick Edwardian at the front and Richard Bickerdyke's 1960s architecture at the rear.

Situated on the city edge, neighbouring two economically-challenged areas of Manchester – Moss Side and Rusholme – they discovered that many local residents felt alienated by the 1908 facade on Oxford Road, finding its formality uninviting, with many not even recognising it as a public art gallery. Despite this, in the year before its closure for renovation, 2012-2103, visitors numbers had still soared from 100,000 to 190,000, with Balshaw describing it as "bursting at the seams".

MUMA's £15m solution addresses these issues by softening Oxford Road entrance, with a gently sloping forecourt that includes a Sculpture Terrace which is open to all. Meanwhile, inside they reconfiguring the existing building to create more public space - gaining an additional 30 per cent - by slightly excavating the ground floor to accommodate the collections which they moved from the Grand Hall, freeing it up to become a function space, and generally improving access for the public by developing alternative routes for artworks and catering.

Moving through the Whitworth from the original entrance is now an enlightening experience, as you walk through from the darker, lower-ceilinged Sixties galleries (the Darbishire Hall, Gulbenkian Gallery and Pilkington Gallery) into the Exhibition Galleries made brighter and loftier by the recovery of the original barrel-ceilings the transition truly lifts the spirits. The central gallery is most appealing with its end opened up to reveal the park beyond, via glazed walkway dubbed the Promenade.

This Promenade which lines the west-facing rear elevation and includes the park entrance, signals the new MUMA terrain, leading off at each end are the two new wings extending into the park. The north-side wing is predominantly brick-built with echoes of Rennie-Mackintosh-meets-Tate Modern when viewed from the Denmark Road, its dense brickwork featuring a "cuff" and pleated elements inspired by the textiles in the Whitworth collection; this houses the Landscape Gallery on the first floor with the Study Centre below.

The most striking wing is on the south side, this long glass box appears to hover lightly in the trees with its cantilevered section at one end; its transparency enhanced by the lack of support columns (the structure cleverly hangs from the roof) and ingenious use instead of polished stainless steel glazing mullions that reflect the park outside. This headline-grabbing element isn't reserved for art, but the Café (operated by The Modern Caterer), the Whitworth no doubt hoping that it will become a destination in its own right since its opening hours run later than the gallery's. It's the element that won MUMA the contract, with their promise of offering visitors the sensation of "dining in the trees" captivating Balshaw and her team.

Below the café is a Learning Studio for school children with folding doors that open out onto the Art Garden, formed by the two new wings and landscaped by Sarah Price, co-designer of the 2012 Gardens at London's Olympic Park.

“We have long held the view that the gallery and park should be a unified experience for our visitors", said Balshaw of the project and there's no doubt that MUMA's renovation places the gallery in context with its natural surroundings and is bound to keep those visitors numbers rising. Before the gallery closed in 2013 it held a celebratory goodbye party for the residents, which led to one attendee called them "the Whitworthies". Well, guess what, they just got worthier.

The Whitworth reopens on February 14 with a Cornelia Parker exhibition