Stanford University is offering $3.4 billion in housing and $1.3 billion for transit and public education benefits as it faces pushback over a proposed 2.3 million-square-foot academic expansion over the next two decades.

The school said in a letter Monday to Santa Clara County that it would spend $3.4 billion to construct 575 affordable housing units and 1,597 market-rate housing units. All the affordable units and 1,015 of the market-rate units would be built first, before 25% of the academic buildings are constructed over the next 20 years.

At least 1,115 of the units and an additional 2,600 beds for student housing would be built on Stanford’s land.

But Joe Simitian, president of the Santa Clara County Board of Supervisors, which must approve the expansion, said the school’s proposal is inadequate.

“If something seems too good to be true, it probably is,” he said. “It doesn’t address the underlying concerns around traffic, housing and open space.”

Stanford’s expansion is expected to lead to 9,610 more people — including students, staff and faculty — on campus each day. Santa Clara County staff called on Stanford to build 2,172 housing units and 2,600 student beds on campus.

Simitian said Stanford’s proposal lacks enough housing in part because it includes the 650-unit Graduate Residences in Escondido Village, which is under construction and set to open in the fall. That development is meant to address existing demand, he said.

The county Planning Commission will consider the project on Thursday and will eventually make a recommendation to the Board of Supervisors, where a final vote could come in the fall.

Stanford said the benefits package is the largest in its history. It comes a week after Google committed to a $1 billion housing package in an effort to build 20,000 housing units. Major Bay Area employers including Facebook, Wells Fargo and Kaiser Permanente have also committed to invest in housing.

“The Stanford community is confronting the serious regional challenges of affordability, housing availability and traffic congestion, and we’re working to do our part to promote solutions that serve Stanford and our neighbors,” Stanford President Marc Tessier-Lavigne said in a statement. “This offer reflects our values as a residential university committed to sustainable development and service to the community.”

The school would also pay $30.3 million in funds for transit improvements and spend $1.1 billion on a transit program. Stanford committed an additional $138.4 million in economic benefits for the Palo Alto Unified School District, including a $15 million innovation space.

Stanford and the school district reached the agreement in April.

“With an average per pupil allocation of $7,050, over the life of our deal, we believe we reached a mutually beneficial agreement through an interest-based bargaining approach. Stanford told us they would be true to their word and they have 100%,” said Don Austin, superintendent of the school district, in a statement.

In April, the county suspended negotiations with Stanford. The two sides have clashed over ordinances that require Stanford to pay affordable housing fees and build 16% of affordable housing in new buildings. The school wants the laws repealed and says it will still meet the requirements in the new proposal.

“So far, the university has not been inclined to fully mitigate the impact of its development,” said Simitian.

UC Berkeley’s expansion plans are also facing opposition.

Chronicle Staff Writer Elena Shao contributed to this report.

Roland Li is a San Francisco Chronicle staff writer. Email: roland.li@sfchronicle.com Twitter: @rolandlisf