Architectural additions to London’s National Gallery have had a troubled history. In the 1980s, plans for an extension to the original building in Trafalgar Square were plentiful. The most notorious was the high-tech Ahrends, Burton & Koralek design of 1984 that triggered the Prince of Wales into insulting it as a “carbuncle on the face of a much-loved and elegant friend” (ungraciously done, at a celebration for the 150th anniversary of the Royal Institute of British Architects). The plan was reversed and an era of reactionary architecture began. Then came the postmodern extension by Venturi Scott Brown, a compromise between the moderns and the classics: now known as the Sainsbury Wing, it was finally completed in 1991, but complications and disputes led to the architects resigning. It has been a story of problems, interference and compromise.

Undeterred, the gallery is having another try. An open international competition is being announced today for significant changes to the 1990s design, to upgrade the Sainsbury Wing and to reimagine the entrance and interiors.

The National Gallery’s director, Gabriele Finaldi, acknowledges the “practically perfect picture galleries” in the Sainsbury Wing but speaks of needing to respond to “a huge increase in visitor numbers and the changing needs and expectations of those visitors over the last 30 years”.

Perhaps wary, in the light of past problems, of going for a concrete plan at this stage, Finaldi tells me, “We haven’t gone out to look for a design but for a designer. We’re still thrilled with the galleries, they always bring great pleasure, the light, the scale. For the 25th anniversary of the building we undertook a restoration and it was wonderful to see how fresh it all still looked.

“But we’re now using the Sainsbury Wing as a main entrance, which it wasn’t intended to be, and we want to ask what the visitor experience should be as they cross the threshold.”

Part of the problem for the National Gallery is that it isn’t just any gallery. It has become the backdrop for the city’s protests, demonstrations and celebrations, a theatrical stage set for London’s psychodramas. It’s a paradoxical place pivotal to the tourist imagination but widely avoided by Londoners themselves.

I wanted to ask Denise Scott Brown, the original architect of the extension with her husband Robert Venturi (who died in 2018), about the plans. Now 89, she sounds a little apprehensive about its future. Snowed in at her Philadelphia home, she replies that she has plenty of time and we speak over the phone.

Scott Brown was born in what is now Zambia and, having spent her childhood in South Africa, she relates how “when I walked through Trafalgar Square on my way to the gallery for visits while we were designing the Sainsbury Wing there were pro-Mandela protests, anti-apartheid banners, and I used to start to cry when I saw them.” It’s a fine indication of the status of this building in not only the British but the global imagination. Trafalgar Square was, after all, named after a naval victory over Napoleon and conceived as the centre of empire, a status it both revels in and contends with in every gesture, from the sculptures on the Fourth Plinth to the buskers and human sculptures who used to clog up the space in front. In the days, that is, when it was almost impossible to imagine that there would, one day, be no one at all in Trafalgar Square.

“There is a famous photo by [Henri] Cartier-Bresson,” Scott Brown says, “portraying a group of people, mostly in raincoats, sitting on a parapet and watching something in Trafalgar Square. And there’s a drunk lying on the ground. Cartier-Bresson seems to have no interest at all in what they’re looking at, he’s only interested in the crowd. But what they’re watching was the coronation of King George VI [in 1937], and Cartier-Bresson doesn’t care. Just like that, we were always as interested in the visitors as in the art.”

As an urbanist, Scott Brown’s approach was to attempt to understand the overlaying of classical bombast, imperial monumentality and intricate medieval street plans, the convergence of London’s entertainment hub of Leicester Square and its representational symbol, Trafalgar Square. “It was city physics,” she says. “Gravity and potential.”

The architecture of the Sainsbury Wing, with its almost fabric-like folds of classical facade, responded to William Wilkins’ 1838 design for the National Gallery itself, but also applied a layer of Mannerism. On top of this, it subsumed elements of Sir John Soane’s Dulwich Picture Gallery and the English establishment’s favourite architect, Palladio. “Palladio and Modernism fight it out on the main facade,” Scott Brown wrote at the time, “and they must respect our era’s discomfort with unalloyed monumentality.” This was an architecturally literate, witty, reflective and subtle design, albeit not without its problems — some of which were forced on the architects. I’ve always struggled, for instance, with the way in which the monumental staircase culminates at a lift door. “There was meant to be a bas relief there,” Scott Brown laments wistfully.

When I ask about the lobby, which is about to be redesigned, she says: “The first problem was that they moved the security machines [bag check] outside into a covered area which we’d intended as a place for Londoners to get out of the rain. So we lost that space.” Regarding the interior, which has been criticised for being dark, grey, even a little oppressive despite its scale, she says: “That was about preparing your eyes. When it opened, people asked if we’d restored the paintings but we hadn’t — it was just that we got visitors acclimatised to a lower light and then when they came across the paintings in the low light conditions they were surprised.”

Some changes have been inevitable. The pedestrianisation of the north side of the square where the gallery is located (by Foster + Partners in 2003) radically changed the patterns of use outside. And the re-emergence of terrorism as a threat (the IRA, remember, was setting off car bombs in London in the 1970s) has changed security requirements. Also, as Finaldi says: “And then there’s Covid. What impact might that have?

“One of the most wonderful things about the building is that we’re right on the street,” says Finaldi. “You can come off Trafalgar Square and be in front of a Holbein in a few seconds. The question is, how do we ensure its openness but also that it remains secure?

“I’d like to keep an open mind,” the director continues. “I hope for some good ideas about how we can make the gallery more responsive.”

The estimated construction cost is £25m-£30m and the plan is to open in time for the National Gallery’s bicentenary in 2024. Whatever they do will probably be criticised. And style will almost certainly come into it again. The Sainsbury Wing is one of the few great mature fruitions of postmodernism — in 2018 it was awarded a Grade I listing for historic buildings — and a design that manages to combine classical tradition, contemporary wit, the modern demands of a big institution and contextuality. To insert something into an already extremely finessed ecology will be tricky, but it will also be intriguing to watch the reactions. Again.