An outdoor amphitheater, public plazas for farmers' markets and a 350-foot-tall tower inspired by a double helix, are among the latest design proposals for Amazon's new headquarters.

The plans, made public and submitted to authorities for approval on Tuesday, will form the second phase of the tech giant's $2.5 billion HQ2 project in Arlington County, Virginia.

More than three years after Amazon announced that it was expanding beyond its current Seattle headquarters, construction at the Virginia site -- located across the Potomac River from Washington, D.C. -- is now well underway. Dubbed PenPlace, the newly unveiled proposal for the project's second phase will provide a further 2.8 million square feet of office space across three 22-story buildings.

The double helix-inspired building will be opened to the public at least two weekends every month, according to Amazon.

The site's focal point will be The Helix, a tree-covered glass structure where a series of "alternative work environments" will be set amid indoor gardens and greenery from the nearby area, tended to by a team of horticulturalists. According to the architecture firm behind the project, NBBJ, a spiral "hill climb" will meanwhile allow employees and visitors to ascend the outside of the structure.

Singapore is building a 42,000-home eco 'smart' city

"We're doing a lot on the site to connect people to nature, said lead architect and NBBJ principal, Dale Alberda, adding that the design aims to symbolize both nature and science. "But with the Helix we really take that to the extreme," he said in a video interview. "We're building a series of indoor atriums and gardens that are not a conservatory or a place you just visit, but a place you can actually go and work."

Public offerings

While the Helix itself will only open to the public occasionally ("at least two weekends" per month, Amazon confirmed to CNN), other parts of the site are intended for use by the community.

The new proposal includes 2.5 acres of public space, offering art installations, communal grassy areas and a 250-seat amphitheater. Outdoor plazas will host mobile food vendors and farmers' markets, while retail space will see shops and restaurants move in at ground level.

According to the project's lead architect, the "vast majority" of ground-level space will be accesible to the public.

"If we do this right, you won't necessarily even know that you're on an Amazon headquarters property," said Alberda, adding that the "vast majority" of the site will be accessible to the public, including office buildings' lobbies.

"People talk about (tech) 'campuses' all the time, and that comes with (the impression of) a place that is fenced off ... but we are moving away from the campus to what we like to refer to as a neighborhood."

Elsewhere, the proposed design features a network of walkways and pedestrianized spaces, and can accommodate over 950 bicycles. Car parking and docking will be pushed below ground, keeping the immediate area free of service and delivery vehicles.

Employees will be able to reach downtown Washington, D.C. -- where Amazon boss Jeff Bezos bought a mansion for a reported $23 million in 2016 -- within 15 minutes by subway.

The proposal features 2.5 acres of public space.

The entire headquarters is expected to run on renewable energy generated at a solar farm approximately 200 miles away in southern Virigina. Other sustainable design features include a system that recycles rainwater and the use of natural ventilation, while the buildings are designed to maximize the amount of sunlight that can enter, thus reduce the amount artificial lighting needed, Alberda said.

Long-term plans

When Amazon first announced plans to build a second headquarters in 2017, it received over 230 proposals from cities and states around the US.

In late 2018, the firm announced that northern Virginia and New York City had both been selected to split the duty as its second headquarters. But a proposal for the latter -- initially set for Long Island City in Queens -- was scrapped months later amid backlash from the local community. At the time, Amazon said that "a number of state and local politicians" in New York had "made it clear that they oppose our presence."

The Helix seen towering above retail facilities.

Virginia on the other hand, has offered generous tax breaks and cash incentives to lure Amazon to the state. The company predicts that its Arlington County headquarters will eventually host 25,000 employees, while it plans to create a further 5,000 jobs at a smaller facility in Nashville, Tennessee.

NBBJ, which is also headquartered in Seattle, has previously designed corporate headquarters for the likes of Samsung and Microsoft. In June last year, the group unveiled plans for a largely car-free "city of the future," equivalent in size to Monaco, for tech giant Tencent in Shenzhen, China.

If Amazon's PenPlace proposal is approved, the project's second phase would break ground in 2022, with construction projected to complete by 2025.