Amazon is offering $20 million to the Arlington County Affordable Housing Investment Fund in exchange for being allowed to build a bigger headquarters complex in the county than zoning allows.

The money is a fraction of what tech companies such as Microsoft, Google and Apple have pledged to address affordable housing concerns on the West Coast, where their presence — and that of Amazon — has sent home prices soaring.

But it would be the greatest single infusion of money ever into Arlington’s housing fund, which in recent years received between $14 million and $16 million annually from the county government and $5 million to $6 million in loan repayments from developers.

The affordable housing proposal has been long awaited by Arlington residents, who worry that the arrival of thousands of well-paid Amazon employees in coming years will greatly ratchet up rents and housing costs. (Amazon founder and chief executive Jeff Bezos owns The Washington Post.)

The company plans to create 25,000 jobs and invest more than $2.5 billion in building its headquarters in the Pentagon City and Crystal City neighborhoods over the next decade. Amazon chose Arlington for its home just over a year ago and has been working to establish ties to the community.

“This opens up a lot of opportunities for us,” Christian Dorsey (D), chair of the Arlington County Board, said of the affordable housing money. “It provides us with the possibility to maybe invest in land for affordable housing. . . . It’s an extremely significant amount.”

He brushed aside comparisons to the money dedicated in Seattle and Silicon Valley by other tech firms. “As long as they’re a healthy company, they will be part of this community for a long time,” Dor­sey said. “They offered this in good faith with what they’ve said all along.”

In exchange for the donation, Amazon wants to increase the allowable size of the 22-story office towers it plans to build at 1400 S. Eads St. and 501 15th St. South from 1.56 million square feet to about 2.15 million square feet.

Among other changes, the company wants to reduce the number of parking spaces, increase the maximum penthouse height and not count the square footage of below-grade equipment and utility rooms, internal atrium areas and above-grade air shafts as part of the overall project density.

The county’s draft site plan amendment, which details the proposal and was released Monday, says Amazon would build a long-sought bike lane on 15th Street South and design, build and maintain an expanded public plaza of about two acres, a project that is estimated to be worth $14 million.

In addition to bus shelters, a bike storage station and improvements to intersections, the company would build an “event space” for up to 700 people that would be available for county-sponsored functions. There would be 65,000 square feet for shops, restaurants and a child-care center. Buildings would be designed to LEED Platinum standards.

The proposal is on the agenda of Monday night’s meeting of the Arlington County Transportation Commission and will be reviewed by the county’s planning commission on Dec. 2 and housing commission on Dec. 5. The Arlington County Board will consider it on Dec. 14 or 17.

In the past, when dealing with other developers, county officials have demanded changes if they determine the density requests are out of line, or if community benefits are not to their liking. Dorsey said that, at first glance, Amazon’s proposal seems “reasonable.”

“It’s not exactly what we were intending, but it is consistent with what we intended,” he said. “From what I’ve heard, the level of density is appropriately mitigated.”

The money for affordable housing is in addition to Amazon’s previously announced $3.3 million for homelessness and housing affordability, as well as $5 million in matching grants for employee donations to local charities this year.

The Arlington County Affordable Housing Investment Fund is a revolving loan fund and the county’s main financing program for affordable housing development. The fund has enabled the county to originate more than $340 million in loans since its inception in 1988.

A total of $14.3 million was allocated to the fund for fiscal 2019, including general fund and recordation tax contributions, as well as loan repayments and developer contributions.