Developers who want to help turn Tysons Corner into “the new downtown” will likely have to pay for most of the related transportation improvements — billions of dollars worth over the next four decades.

For years Fairfax County has been working on plans to transform Tysons Corner from a sprawling office park to a walkable urban center. The effort culminated with the adoption of an ambitious guiding document in 2010 that calls for the area to be largely rebuilt.

An eastward-looking view of the Silver Line under construction as it passes along Leesburg Pike through Tysons Corner near Route 123 on Sept. 24. (Jahi Chikwendiu/WASHINGTON POST)

Created over 18 months by members of the county’s planning commission, the funding proposal calls for developers — rather than existing commercial landowners and residents — to bear the bulk of the costs. In addition to building parks, athletic fields and community centers, developers would be required to pay for a new grid of streets as well as major road projects.

In total, the plan calls for more than $1 billion to come from sources funded exclusively by developers.

“I think this represents the outermost edge of what we can realistically ask developers to pay,” said Walter Alcorn, a planning commissioner who led the team that created the proposal. “It’s aggressive.”

But that doesn’t mean existing landowners would be off the hook. The plan also includes a special tax district expected to raise $250 million over the next 40 years. The exact boundaries and rate have yet to be set, though examples in the plan suggest that the property tax would be between 7 and 9 cents per $100. The proposal recommends that residential landowners, who account for roughly 10 percent of Tysons’ assessed value, be exempted, leaving only commercial landowners, such as the malls, to pay the tax.

That means people who live in Tysons would pay no more than other county residents, who would chip in through existing taxes — general fund dollars — and new bonds.

The county will also seek some state and federal money.

The proposal breaks the expenses into categories and lays out who, theoretically, should pay for each: Public transit projects (Metro not included), neighborhood access improvements and major roads leading to Tysons should be the public’s responsibility. A new grid of streets and major roads inside Tysons should be paid for privately.

The Board of Supervisors is expected to support the proposal. The county will hold a public hearing on the matter Oct. 16, with a vote in the following weeks.

The 2010 master plan aims to remake Tysons into a walkable, vibrant downtown over the next several decades. An estimated 100,000 people work in Tysons, and about 18,000 live there. The master plan envisions a city that will be home to 100,000 residents and 200,000 workers by 2050, all anchored by Metro.

Four Silver Line stations are being built in Tysons. The county wants three quarters of future growth to be within a half-mile of the stops.

So far, the county has approved two major development applications that it has deemed in keeping with the new vision for Tysons, most recently a massive expansion of Capital One’s headquarters. More than a dozen other applications for Tysons remain under county review. All of them will be subject to the transportation funding plan.

As for the branding plan, not all supervisors agreed that Tysons should be marketed as “the new downtown,” as suggested by the experts who came up with the idea.

Gerry Hyland (D-Mount Vernon) said he worried that such definitive language might spark resentment in other parts of the county or region. His suggestion? A new downtown.

One thing everyone seemed to agree on: Corner is out.

Henceforth, it’s just Tysons.