Work is nearing completion on Jamie Fobert Architects’ cliffside extension of Cornwall’s Tate+St+Ives'>Tate St Ives, with an official opening date now set for 14 October this year.

A four-year construction project, the plans go back 12 years to 2005 when the London-based Jamie Fobert was first appointed to double the size of the museum.

In addition to lengthy delays because of planning issues, Jamie Fobert lost the project in 2011 when the site was changed, but then reclaimed it after winning a second competition in 2012 for the modified site. BAM Construction is carrying out the redevelopment works.

Adding 600sq m (6,460sq ft) of new gallery space, the four-storey extension gives the gallery ample space to accommodate its 250,000 annual visitors – a figure more than three times the numbers the building was originally designed for. The expansion also means Tate St Ives will be able to stay open year-round for the first time, without the need to temporarily close while new exhibitions are installed.

The new gallery, sunk into the cliff alongside the original building, will offer artists and curators a column-free space lit by six large skylights. A public garden on the roof will be connected to the cliff above and the beach below.

The redevelopment also features a new collection care studio, loading bay, staff offices and improved visitor facilities. The building itself is clad in handmade ceramic tiles with a blue-green glaze, which have been designed to reflect the changing colours of the sky and sea.

In addition to the work of Jamie Fobert, Evans and Shalev – the architects of the original Tate St Ives building – have also played a part in the redevelopment, adding new space for hands-on workshops and family activities, a ground-floor studio for visitors to explore archival and digital material, and a studio on the roof terrace with views looking out over the sea. The existing galleries have also been fully refurbished and are being integrated into these additions.

Originally budgeted at £12m (US$15.3m, €14.1m), the now £20m (US$25.6m, €23.5m) project has been made possible through funding from a number of public sources including Cornwall Council, Arts Council England (ACE), the Coastal Communities Fund, the Heritage Lottery Fund (HLF) and the DCMS/Wolfson Museums and Galleries Improvement Fund.

Several private organisations and donors have also contributed, including a major gift from The Headley Trust, and donations from the Clore Duffield Foundation, The Foyle Foundation, The Ronald and Rita McAulay Foundation, Lord and Lady Myners of Truro, and the Garfield Weston Foundation.

Once open, the original building will be dedicated to a display exploring modern art in St Ives and its relationship with the wider world. The new galleries for seasonal exhibitions will open with Rebecca Warren, an artist renowned for her clay sculpture pieces.