John Lewis is to spend £20m on a revamp of its Glasgow store in the city centre’s Buchanan Galleries in a vote of confidence in the shopping mall not long ago scheduled for demolition.

It is the largest cash injection within a wider plan to spend £50m this financial year on refreshing its shops, with department stores in Reading, Cambridge, Leicester and Liverpool all earmarked for an upgrade.

The Glasgow project includes expanding the beauty hall with a fragrance hall and gift emporium. The company said every corner of the 28,000 sq metre store would be upgraded, including a new John Lewis Platter in-house cafe-restaurant and more women’s and men’s fashion labels.

The lower ground floor will be the first to be completed, with a technology and sports floor due to be in place by late September. The store will remain open throughout, with the refurbishment expected to finish early next year.

The leader of Glasgow city council, Susan Aitken, said: “John Lewis has been one of the city centre’s key shopping attractions for almost 30 years and this huge vote of confidence in Glasgow’s retail sector will, I’m sure, guarantee it continues to be for at least another 30.”

The refurbishment of the Buchanan Galleries’ flagship tenant comes after the mall’s owner, LandSec, won planning permission last year for a major overhaul of the 1990s shopping centre, having ditched proposals to demolish it to build offices in 2024.

LandSec has bought up neighbouring buildings for the renovation plan, which will create new leisure and dining space, including a food hall, as well as new larger retail spaces on Buchanan Street.

Other new developments in the city include the former Watt Brothers department store, a vacant historical building for which permission was recently granted for it to be converted into a hotel.

Last year’s 850th anniversary for the city was seen as a moment to try to revive the centre, which has been hit hard by online shopping and the decline of some of its historical buildings, including the burned-out Glasgow School of Art’s Mackintosh building and the Centre for Contemporary Arts, which permanently closed in January after serious concerns were raised about its finances.

Earlier this year, services via Glasgow Central station were disrupted after a fire gutted a neighbouring Victorian building, and the centre is scarred with vacant sites, such as the former BHS, and stalled renovations.

John Lewis’s £50m outlay is the latest phase of the 161-year-old retailer’s programme to spend £800m by 2029 on rebooting its 36 branches.

It has been introducing new brands with cross-generational appeal, from Topshop and Carhartt in fashion, to Charlotte Tilbury in beauty, to Waterstones bookshops.

Peter Ruis, the managing director of the employee-owned chain, said he was “getting rid of the old stuffy department store and replacing it with something more experiential”.

The investment is part of attempts to revive the department store as rivals such as Debenhams and Beales have disappeared from high streets, while House of Fraser has more than halved in size.

Sales at John Lewis’s department stores increased by 3% to £4.9bn in the year to 31 January, with underlying profit up 29% to £58m.

That helped clear the way for the parent group, the John Lewis Partnership, which also owns Waitrose, to pay a bonus to staff for the first time in four years.