Apple's new 'spaceship' campus might have cost as much as US$5 billion ($7.3b), according to Santa Clara County's tax assessor.

Assessor Larry Stone told the San Jose Mercury News that he estimated the cost for the California campus to be the figure, the MailOnline reported

If true, the number is significantly higher than previous estimates.

Construction company BuildZoom, for example, estimated that the 70.8ha campus cost US$1.1b.

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Stone told the Mercury News that the cost discrepancy could be attributed to misunderstandings about building permits and not fully accounting for labor costs.

Apple has not publicly commented on the cost of the campus.

A final assessed property value has not yet been done.

In September 2016, the Assessor's Office estimated that the Apple spaceship had, as of January 2016, cost US$1.6b.

Stone also discussed how 'unique and innovative' Apple Park is, which could affect its final value.

For comparison, Levi's Stadium in Santa Clara and the SalesForce Tower in San Francisco cost, respectively, $1.3b and $1b.

Apple revealed the Visitor's Center for Apple Park, which has its own cafe and Apple Store, this past November. It is directly opposite the new HQ.

Visitors are able to see the firm's spaceship from a rooftop viewing deck.

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Drone footage from September shows the state-of the art complex.

Shot by drone pilot Duncan Sinfield, the footage shows a nearly complete main 'spaceship' structure, along with landscaping work inside the main ring of the building.

It's expected the final tree count will be around 9,000 and will include apple, apricot, and cherry trees.

An underground auditorium will be where Apple's CEO Tim Cook will present the companies keynotes ahead of product launches. Picture / AP
An underground auditorium will be where Apple's CEO Tim Cook will present the companies keynotes ahead of product launches. Picture / AP

The drone images also show the finished Steve Jobs Theater, named after the late visionary leader of Apple, sitting atop a man-made hill with a glass exterior foyer.

One video shows the exterior of the spaceship, the sleek walls of the new theatre, the 'Apple Campus tunnel,' and several areas still under construction.

In subsequent videos, both interior and exterior walls of the 360-degree curved glass office building, which has been nicknamed 'spaceship, can be seen.

The building also features a two-story yoga room, a 4,000 person cafe with four-story glass doors, and even patented pizza boxes to prevent food from getting soggy.

Appearing as a giant saucer, the Silicon Valley site near the 280 Highway will be home to 13,000 Apple employees.

There will be jogging and cycling trails, with more than a thousand bikes kept on site at all times, which staff can use to make their way around.

The Spaceship will have 360-degree curved glass fronted walls and central courtyard as well as a 1,000-seater auditorium, a gym and 27,871 sqm of 'research' space.

Apple Campus 2 will additionally have underground parking hidden from view, meaning 80 per cent of the site can be covered in trees. The site was previously owned by Hewlett Packard and the majority of the area is currently covered in asphalt.

Elsewhere underground, the auditorium will be where Apple's CEO Tim Cook will present the companies keynotes ahead of product launches, for example. This auditorium will be covered with a circular glass pavilion that will also be an access point for employees and guests.

Natural gas will, primarily, provide the building's power and the local energy grid will only be accessed in emergencies.

The glass structure will also be fitted with solar panels.

The circular, four-storey building will be around a mile in circumference and a third of a mile wide and was recently described by the San Francisco Weekly as a 'massive glass doughnut'.

The building stays eco-friendly with natural ventilation that works instead of air-conditioning for 70 per cent of the year, low energy LED lighting where natural light doesn't reach, and on-site recycling.