The top 85 floors of Jeddah's under-construction marvel, Kingdom Tower, will go unoccupied as they are too narrow for apartments or offices, international reports suggest.

UK daily The Guardian reported: "While the Kingdom Tower promises to house 121 serviced apartments, 360 residential apartments, a 200-room Four Seasons hotel and much office space to boot, its top 85 floors will go unoccupied as they are too narrow for apartments or offices.

"Some (of those floors) will house plant machinery and communications equipment," the report continues.

Upon completion, Kingdom Tower will be at least 1km tall, thus stealing the crown of the world’s tallest building from Dubai’s 828m-tall Burj Khalifa.

The Guardian's report adds Kingdom Tower's developers will not be too perturbed by how long the tower takes to "fill up".

"In truth, the developers are probably not that bothered if their tower takes time to fill up – this is often the case when exceptionally tall, speculative towers are marketed.

"The Empire State Building took two decades to fill with tenants," the report adds.