General Motors Co. is postponing renovations to its famed design studios and halting a project to update its global propulsion headquarters in metro Detroit an effort to save money.

The plans for the projects in Pontiac and Warren, which were expected to cost hundreds of millions of dollars, were detailed in a long letter sent Wednesday to employees by GM CEO Mary Barra, making the case that the Detroit-based automaker needs to curtail costs to protect itself against an unpredictable future. A copy of the letter was obtained by Automotive News.

The two projects Barra cited include $180 million for a 360,000-square-foot expansion to its design studios and a new parking structure. Both were supposed to be the "final stage" of a $1 billion transformation of GM's tech center campus in Warren.

Renovations at the propulsion center in Pontiac were part of a wider initiative by the automaker to update and transform its older facilities into open office spaces.

It's unclear how much of the planned renovations were completed. A GM spokesman said those details weren't immediately available.

The letter also detailed plans to offer voluntary buyouts to about 18,000 salaried employees in North America and "most" global executives who have 12 years or more experience, which the automaker confirmed Wednesday.

In the letter, Barra details internal and external factors such as trade and global economic conditions that the company cannot control but must be prepared to address.