UPDATE 8/3/20: I posted this in 2016. A number of people have stated this no longer works so please use at your own risk. I am not working in iOS at the moment so I do not have an update handy. Best of luck!
Here is one way to do it:
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
let height: CGFloat = 50 //whatever height you want to add to the existing height
let bounds = self.navigationController!.navigationBar.bounds
self.navigationController?.navigationBar.frame = CGRect(x: 0, y: 0, width: bounds.width, height: bounds.height + height)
}