Over 70 architects, critics and preservationists have signed a petition to prevent the "tremendous mistake" of demolishing parts of Robert Venturi and Denise Scott Brown's Museum of Contemporary Art San Diego, to make way for an expansion by Selldorf Architects.

Architects Terry Farrell, Robert AM Stern and Toshiko Mori, critic Paul Goldberger, curator Martino Stierli, and historian Charles Jencks are among those campaigning to save the museum (MCASD) in the village of La Jolla.

They argue that the current plans will compromise the 1996 renovation and extension, by Venturi Scott Brown & Associates (VSBA) with David Singer Architect, destroying its facade and entrance courtyard, described as "a well-loved urban space".

"The Museum of Contemporary Art San Diego stands on the verge of making a tremendous mistake: demolishing much of its landmark La Jolla building designed by Denise Scott Brown and Robert Venturi," said the petition.

"We recognise the museum's need to expand, but we ask that it do so without irreparably damaging a cultural landmark and in the process severely weakening La Jolla's beloved village centre."

Extension would remove VSBA's colonnade and courtyard

Since 1941, the museum has occupied Scripps House – a 1915 villa designed for philanthropist Ellen Browning Scripps by American architect Irving Gill – which has been extended multiple times.

The building overlooks the Pacific Ocean on one side and faces onto the lively Prospect Street thoroughfare on the other. VSBA's expansion included a courtyard fronted by a colonnade along the street, which currently leads to the museum's primary entrance, directly into the original Scripps House.

These elements would be removed as part of Selldorf Architects' proposal, which involves increasing current gallery space from 10,000 to 40,000 square feet (930 to 3,700 square metres) at the southern end of the site, creating a new entrance, and enhancing the site's connection to the nearby ocean.

The latest extension is designed as a series of small volumes "to balance the scale of the new addition with the existing structure", according to the firm's website.

Scott Brown and Jencks plead for plans to be revised

But Scott Brown does not agree that Selldorf Architects' vision for the project matches the sensitivity her firm took with the 1996 extension.

"In designing our building, we carefully analysed and reacted to a pattern of activities on Prospect Street," she said in a statement sent to Dezeen along with the petition. "But now the delicate connections that we created are to be severed, equally threatening the museum and the village. Why not go on from what we so lovingly provided?"

The petition carrying 67 signatures was sent to the museum's board chairman Paul Jacobs, and director and CEO Kathryn Kanjo, on 27 July 2018. Eight more had been added at the time of writing.

Architectural historian and postmodernism theorist Jencks also issued a statement to accompany the letter, describing the current museum as a "Time City" where one period of the building's history acknowledges another.

"Irving Gill's original and VSB's creative addition talk across time in related languages to create a greater whole," said Jencks.

"You have the perfect beginning of such a dialogue underway in La Jolla," he continued. "For heaven's sake don't destroy or degrade its qualities – build on them, ask VSB for their suggestions, please address your obligations to the difficult but more enjoyable whole."

The petition similarly urges MCASD to consult with VSBA on a revised proposal that complements the firm's design.

"We ask that the museum do better," it said. "We ask that it reconsider the value of its existing building and come up with a plan for expansion that is sensitive and respectful to the village of La Jolla."

MCASD defends Selldorf's plans

However, the museum released a counter statement explaining how the new entrance will improve the visitor experience.

"With the Venturi Scott Brown & Associates' columned courtyard, guests were consistently unable to locate the entrance, gravitating either to the shuttered Gill doorway or to the southern auditorium entry," said the statement.

"Following the building's 1996 opening, at the behest of the museum, VSBA made an effort to remedy this important issue by designing additional exterior and interior signage," it continued. "Despite these efforts, the entry to the museum remained elusive to many visitors."

MCASD also argued that Selldorf's extension will enhance all phases of the building, including Gill's original, so visitors can better understand its evolution.

"VSBA's work is not being destroyed; yes, the columns and the pergola are being removed but the vast majority of their contribution will remain," MCASD's statement added.

VSBA's legacy should be preserved, says petition

Venturi and Scott Brown are widely considered as instigators of the postmodern architecture movement, which developed during the second half of the 20th century as a reaction against modernism.

Venturi was awarded the 1991 Pritzker Prize in recognition of his contribution to the field, and many high-profile figures recently campaigned for Scott Brown to be added to the accolade for her part in their firm's output – albeit unsuccessfully.

The duo were jointly named as the 2016 AIA Gold Medal laureates, and Scott Brown was awarded the 2017 Jane Drew Prize for raising the profile of women in architecture.

"Scott Brown and Venturi are widely recognised as among the 20th century's most important architects, and this building remains a shining reminder of their enduring cultural contribution," said the letter to MCASD.

Founded by architect Annabelle Selldorf, New York-based Selldorf Architects has completed several renovations and expansions of cultural buildings, including the Clark Art Institute in Massachusetts with Tadao Ando.

However, the firm is currently facing controversy over another museum project. Its expansion of the Manhattan's Frick Collection gained planning approval last month, despite protests and opposition from preservationists.

Dezeen contacted Selldorf Architects for comment on the MCASD petition, but had not received a response at the time of publishing.

Campaigns are also ongoing to prevent interventions of postmodernist landmarks elsewhere in the US. They include Philip Johnson's AT&T Building in New York, which is undergoing a renovation by Snøhetta, and Charles Moore's Hood Museum of Art in New Hampshire, set for a major overhaul by Tod Williams Billie Tsien Architects.