How to increase the UITableView separator height?

The best way for me, just add this in cellForRowAtIndexPath or in willDisplayCell

CGRect sizeRect = [UIScreen mainScreen].applicationFrame;    
NSInteger separatorHeight = 3;
UIView * additionalSeparator = [[UIView alloc] initWithFrame:CGRectMake(0,cell.frame.size.height-separatorHeight,sizeRect.size.width,separatorHeight)];
additionalSeparator.backgroundColor = [UIColor grayColor];
[cell addSubview:additionalSeparator];

For Swift 3.0:

let screenSize = UIScreen.main.bounds
let separatorHeight = CGFloat(3.0)
let additionalSeparator = UIView.init(frame: CGRect(x: 0, y: self.frame.size.height-separatorHeight, width: screenSize.width, height: separatorHeight))
additionalSeparator.backgroundColor = UIColor.gray
self.addSubview(additionalSeparator)

You should add this to cell’s method awakeFromNib() to avoid re-creation.