Apple's spaceship HQ valued as one of the world's most expensive buildings

The firm paid $6m into an affordable housing fund but critics say it must do more to help solve Silicon Valley's social issues

Apple's donut-shaped headquarters in Cupertino, California has been valued at $4.17bn, according to a new assessment this week - making it one of the most expensive buildings in the world.

David Ginsborg, deputy assessor of Santa Clara county, where Cupertino is located, said that the unconventional 2.8m sq ft property was a challenge to evaluate.