I have an app where the UITableView\'s separator inset is set to custom values - Right 0, Left 0. This works perfectly in iOS 7.
I believe this is the same question that I asked here: Remove SeparatorInset on iOS 8 UITableView for XCode 6 iPhone Simulator
In iOS 8, there is one new property for all the objects inherit from UIView. So, the solution to set the SeparatorInset in iOS 7.x will not be able to remove the white space you see on the UITableView in iOS 8.
The new property is called "layoutMargins".
@property(nonatomic) UIEdgeInsets layoutMargins
Description The default spacing to use when laying out content in the view.
Availability iOS (8.0 and later)
Declared In UIView.h
Reference UIView Class Reference

The solution:-
-(void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath{
if ([tableView respondsToSelector:@selector(setSeparatorInset:)]) {
[tableView setSeparatorInset:UIEdgeInsetsZero];
}
if ([tableView respondsToSelector:@selector(setLayoutMargins:)]) {
[tableView setLayoutMargins:UIEdgeInsetsZero];
}
if ([cell respondsToSelector:@selector(setLayoutMargins:)]) {
[cell setLayoutMargins:UIEdgeInsetsZero];
}
}
If you set cell.layoutMargins = UIEdgeInsetsZero; without checking if the layoutMargins exists, the app will crash on iOS 7.x. So, the best way would be checking if the layoutMargins exists first before setLayoutMargins:UIEdgeInsetsZero.