Work is due to begin in January on Saudi Arabia’s ambitious 1km-tall Kingdom Tower, which is planned to surpass the UAE’s Burj Khalifa as the tallest building in the world.

Speaking to Abu Dhabi-based The National newspaper, the project’s lead architect Adrian Smith said the project is “moving rapidly,” adding that the architecture firm had “actually advanced the [building’s] design to a high level of detail.”

The project, which will be used for residential, office and retail space, is estimated to cost $1.2 billion, and will include a six-floor “sky palace” occupying the 158th-164th floors.