How to change height of grouped UITableView header?

Return CGFLOAT_MIN instead of 0 for your desired section height.

Returning 0 causes UITableView to use a default value. This is undocumented behavior. If you return a very small number, you effectively get a zero-height header.

Swift 3:

 func tableView(_ tableView: UITableView, heightForHeaderInSection section: Int) -> CGFloat {
        if section == 0 {
            return CGFloat.leastNormalMagnitude
        }
        return tableView.sectionHeaderHeight
    }

Swift:

func tableView(tableView: UITableView, heightForHeaderInSection section: Int) -> CGFloat {
    if section == 0 {
        return CGFloat.min
    }
    return tableView.sectionHeaderHeight
}

Obj-C:

    - (CGFloat) tableView:(UITableView *)tableView heightForHeaderInSection:(NSInteger)section
{
    if (section == 0)
        return CGFLOAT_MIN;
    return tableView.sectionHeaderHeight;
}

This worked for me with Swift 4. Modify your UITableView e.g. in viewDidLoad:

// Remove space between sections.
tableView.sectionHeaderHeight = 0
tableView.sectionFooterHeight = 0

// Remove space at top and bottom of tableView.
tableView.tableHeaderView = UIView(frame: CGRect(origin: .zero, size: CGSize(width: 0, height: CGFloat.leastNormalMagnitude)))
tableView.tableFooterView = UIView(frame: CGRect(origin: .zero, size: CGSize(width: 0, height: CGFloat.leastNormalMagnitude)))

If you use tableView style grouped, tableView automatically set top and bottom insets. To avoid them and avoid internal insets setting, use delegate methods for header and footer. Never return 0.0 but CGFLOAT_MIN.

Objective-C

- (CGFloat)tableView:(UITableView *)tableView heightForHeaderInSection:(NSInteger)section {
    // Removes extra padding in Grouped style
    return CGFLOAT_MIN;
}

- (CGFloat)tableView:(UITableView *)tableView heightForFooterInSection:(NSInteger)section {
    // Removes extra padding in Grouped style
    return CGFLOAT_MIN;
}

Swift

func tableView(_ tableView: UITableView, heightForHeaderInSection section: Int) -> CGFloat {
    // Removes extra padding in Grouped style
    return CGFloat.leastNormalMagnitude
}

func tableView(_ tableView: UITableView, heightForFooterInSection section: Int) -> CGFloat {
    // Removes extra padding in Grouped style
    return CGFloat.leastNormalMagnitude
}