A historic fine art museum in Nantes, France, will re-open to the public tomorrow (23 June) following a major transformation by British architecture practice Stanton Williams.

The firm have designed several extensions to the Musée d'arts de Nantes’ original 19th Century ‘Palais’ building and 17th Century Oratory Chapel – creating an additional 4,000sq m (430,000sq ft) of space for artworks to be exhibited.

The €48.8m (US$545m, £430m) project was conceived to turn the popular institution – previously known as the Musée des Beaux-Arts – from an “introverted [complex] into a vibrant, democratic and welcoming contemporary space that is open to the city and its people and visitors.”

New facilities include an auditorium, library, educational spaces, an archive, and an external sculpture court. New landscaping and access routes have also been added around the museum.

The Palais, designed in the 1890s by Nantes-based architect Clément-Marie Josso, has been linked by a new gallery to an extension called the ‘Cube’. This features contemporary galleries on four floors. Openings to the outside present the cityscape as another artwork – “offering a reversed extension of space and carrying the imprints of the pedestrians, cyclists, and cars, as they journey around.”

The staircase on the south side of the extension is formed of a suspended curtain wall made of marble and translucent laminated glass; a light contrast to the Palais’ existing protected monumental staircase. The glass roof that used to light the galleries has been replaced by superimposed layers of glass and stretched fabric – creating a ‘passing clouds’ effect to retain and optimises natural light.

To pay tribute to the Beaux-Arts architecture of the original Musée, Stanton Williams used a consistent palette of materials in the renovated spaces.

“This creates the impression that the museum is one monolithic volume - carved out of a single block of stone,” they said in a statement.

Patrick Richard, a director at the practice and project leader, said: “We’re grateful to the city of Nantes for entrusting us to transform and extend the City's Museum of Art, as well as designing the exhibition spaces for their exceptional and valuable art collections, providing flexible galleries for temporary art installations.

“The new museum has been conceived around the existing collections; creating an intimate dialogue between art and architecture that firmly embeds it within the distinctive historical setting of the city.”

As part of the inaugural exhibition programme at the relaunched museum, Stanton Williams have curated a six-month long show detailing the processes involved in the making of a significant public museum.