this is quite old. Nevertheless I will post my approach.

Simply increase your cell height a bit and assign a mask layer to the cell, like that:

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "...", for: indexPath)

    // Configure the cell...
    let maskLayer = CAShapeLayer()
    let bounds = cell.bounds
    maskLayer.path = UIBezierPath(roundedRect: CGRect(x: 2, y: 2, width: bounds.width-4, height: bounds.height-4), cornerRadius: 5).cgPath
    cell.layer.mask = maskLayer

    return cell
}

So in this example my seperator height will be 4.

Have fun!

Swift 4

It's not possible to make the default separator higher. Instead you need to add a subview that will look as a separator to each cell (and optionally make the cell higher). You can do it for example in cellForRowAtIndexPath or in a UITableViewCell subclass.

In case you allow to select the cell, you need to add the subview for selected state as well, otherwise the separator would disappear when the cell is selected. That's why selectedBackgroundView is also configured.

Add this into your UITableViewController subclass:

override func viewDidLoad() {
    super.viewDidLoad()
    tableView.separatorStyle = .none
}

override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "cell", for: indexPath)

    cell.backgroundView = UIView(backgroundColor: .white)
    cell.backgroundView?.addSeparator()

    cell.selectedBackgroundView = UIView(backgroundColor: .blue)
    cell.selectedBackgroundView?.addSeparator()

    // configure the cell

    return cell
}

Add this extensions into the same file at the bottom:

private extension UIView {
    convenience init(backgroundColor: UIColor) {
        self.init()
        self.backgroundColor = backgroundColor
    }

    func addSeparator() {
        let separatorHeight: CGFloat = 2
        let frame = CGRect(x: 0, y: bounds.height - separatorHeight, width: bounds.width, height: separatorHeight)
        let separator = UIView(frame: frame)
        separator.backgroundColor = .gray
        separator.autoresizingMask = [.flexibleTopMargin, .flexibleWidth]

        addSubview(separator)
    }
}

Here's an option that might work for some people

cell.layer.borderColor = UIColor.white.cgColor
cell.layer.borderWidth = 4.0
cell.layer.masksToBounds = true

The easier and safest solution to this problem is to turn off the table separator and use a UITableViewCell as a separator of variable height. Sure, you'll have to do some index math to figure out where items are, but really it's odd / even.

It won't break and you get the benefit of recyclable cells (no extraneous views to clean up).

First make tableview separator none from the storyboard. Then add UILabel/UIView at bottom of cell of height(you needed) using storyboard or Xib

For Swift 4

Implemented King-Wizard's solution to Swift 4:

public func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
    let additionalSeparatorThickness = CGFloat(4)

    let additionalSeparator = UIView(frame: CGRect(x: 0,
                                                   y: cell.frame.size.height - additionalSeparatorThickness, width: cell.frame.size.width, height: additionalSeparatorThickness))
    additionalSeparator.backgroundColor = UIColor.groupTableViewBackground
    cell.addSubview(additionalSeparator)

}

This is the easiest solution I've found:

override func tableView(_ tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
    " "
}

then just set the height to whatever you want:

tableView.sectionHeaderHeight = 30.0