News: Peter Zumthor has drastically modified his plans to overhaul the Los Angeles County Museum of Art, to prevent damaging the nearby tar pits that inspired his design.

The Swiss architect first unveiled his proposal to demolish the four buildings of the Los Angeles County Museum of Art (LACMA) one year ago, to make way for a new $650 million (£380 million) complex with an undulating black form referencing the La Brea Tar Pits.

Although few objected to the demolition of the existing buildings – only 424 people have signed the online petition – the neighbouring Page Museum raised concerns that the tar pits could be damaged during construction, and would be thrown into shadow by the new structures.

Zumthor has responded by pulling the complex back. However, to achieve the required gallery space without comprising the low-rise design, it will now have to extend over Wilshire Boulevard – one of LA's biggest and busiest roads.

This south-western extension will allow the building to meet a patch of land already owned by LACMA at the corner of Spaulding Avenue and Wilshire, currently used as a car park.

Jane Pisano, director of the Natural History Museum that runs the Page Museum, told the New York Times that that original design would have "severely impacted six of the nine active tar pits", but that her board fully supports the reworked design.

"We are so pleased," said Pisano. "I do believe this design direction preserves and protects the tar pits."

Other notable changes include new holes in the roof, opening cafe and staircase areas up to the sky, and the removal of a glazed wall that would have allowed pedestrian visitors to come face-to-face with people arriving by car.

As before, parts of the structure will be raised above ground level on nine-metre-high glass columns, providing ground-level storage for artworks.

LACMA director Michael Govan told the New York Times that the proposal will still have the same 37,000-square-metre area. It "doesn't change our basic vision and has the added benefit of lightening the mass in the park," he added, referring to the campus north of Wilshire.

Los Angles mayor Eric Garcetti has come out in favour of the new design. "I think this is a bridge in many ways," he said. "It opens up the fortress that is LACMA to the world" and allows "pedestrian and car cultures to coexist in an exciting new way".

However Zumthor's latest scheme has been criticised by LA Times critic Christopher Hawthorne, who describes it as "the product not so much of boldness as a combination of diplomacy and fidelity — perhaps misguided — to certain elements of his original design".

"The more I think about the plan's newly attenuated form, stretched like a piece of black bubble gum across Wilshire, the more I wonder if the architect's basic reading of Los Angeles could use an update," he said.

Zumthor has been working on the project for over six years, but designs were only revealed to the public in June 2013.

Images courtesy of Atelier Peter Zumthor & Partner.