Trying to create an iOS WKWebView with a size smaller than the screen programmatically

Im starting with a simple app that just shows a web view using WKWebView. Everything builds, and works okay, except that it is always fullscreen. Ideally it would not extend under the iOS status bar at the top. It needs to be lowered by 20px to be below it. I have searched extensively for a solution but nothing changes the size. Nothing is setup in InterfaceBuilder, I'm doing everything programmatically. Using Swift.

This seems like something basic that many apps with a WKWebView would do. It should be simple. I'm probably missing something obvious. Any help is appreciated. Thanks in advance.

This is what I have so far:

import UIKit
import WebKit
class ViewController: UIViewController, UIGestureRecognizerDelegate , UIWebViewDelegate, WKNavigationDelegate  {

    let url = NSURL(string:"http://stackoverflow.com")
    var webView: WKWebView!

    override func viewDidLoad() {
        super.viewDidLoad()
        webView = WKWebView(frame: CGRect( x: 0, y: 20, width: 380, height: 150 ), configuration: WKWebViewConfiguration() )
        self.view = webView
        self.view.frame = webView.frame
        let req = NSURLRequest(URL:url!)
        webView.loadRequest(req)
        self.webView.allowsBackForwardNavigationGestures = true
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}

Many thanks Emilio!! The adjusted code works perfectly!

import UIKit
import WebKit
class ViewController: UIViewController, UIGestureRecognizerDelegate , UIWebViewDelegate, WKNavigationDelegate  {

    let url = NSURL(string:"http://stackoverflow.com")
    var webView: WKWebView!

    override func viewDidLoad() {
        super.viewDidLoad()
        webView = WKWebView(frame: CGRect( x: 0, y: 20, width: self.view.frame.width, height: self.view.frame.height - 20 ), configuration: WKWebViewConfiguration() )
        self.view.addSubview(webView)
         let req = NSURLRequest(URL:url!)
        webView.loadRequest(req)
        self.webView.allowsBackForwardNavigationGestures = true
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}