Issues setting gradient image as UINavigationBar background

I'm trying to set a gradient image background for my navigation bar. Code is like so:

extension CAGradientLayer {

    class func gradientLayerForBounds(bounds: CGRect, startColor: UIColor, endColor: UIColor) -> CAGradientLayer {
        let layer = CAGradientLayer()
        layer.frame = bounds
        layer.colors = [startColor.CGColor, endColor.CGColor]
        return layer
    }
}

Then in my view controller:

func imageLayerForGradientBackground() -> UIImage{

        var updatedFrame = self.navigationController!.navigationBar.bounds
        updatedFrame.size.height += 20 //Status bar
        let layer = CAGradientLayer.gradientLayerForBounds(updatedFrame, startColor: UIColor(hexString: "5FD238")!, endColor: UIColor(hexString: "A5E589")!)
        UIGraphicsBeginImageContext(layer.bounds.size)
        layer.renderInContext(UIGraphicsGetCurrentContext()!)
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return image

    }

viewDidLoad:

   navigationController!.navigationBar.translucent = false
   navigationController!.navigationBar.tintColor = UIColor.whiteColor()
    navigationController!.navigationBar.setBackgroundImage(imageLayerForGradientBackground(), forBarMetrics: UIBarMetrics.Default)

The issue is that all my views under the navigation bar are now pushed down by what look like the height of the navigation bar (as drawn badly in the diagram below):

Why is this happening? Any pointers on this would be greatly appreciated! Thanks!

EDIT:

func setupDrawerView() {

        answerDrawerView = AnswerDrawerView(frame: CGRect(x: 0, y: UIScreen.mainScreen().bounds.size.height - AnswerDrawerView.submitTabHeight, width: UIScreen.mainScreen().bounds.size.width, height: UIScreen.mainScreen().bounds.height*0.75))
        answerDrawerView.delegate = self
        view.addSubview(answerDrawerView)
    }