Add extra field to check-out

/**
 * Add an extra field in checkout (VAT)
 */
function twstudio_custom_override_checkout_fields( $fields ) {
    $fields['billing']['billing_vat'] = array(
        'label'     => __('VAT', 'woocommerce'),
        'placeholder'   => _x('', 'placeholder', 'woocommerce'),
        'required'  => false,
        'class'     => array('form-row-wide'),
        'clear'     => true
    );
    return $fields;
}
add_filter( 'woocommerce_checkout_fields' , 'twstudio_custom_override_checkout_fields' );

// Display field value (VAT) on the order edit page
function custom_checkout_field_display_admin_order_meta($order){
    echo '<p><strong>'.__('VAT').':</strong> ' . get_post_meta( $order->get_id(), '_billing_vat', true ) . '</p>';
}
add_action( 'woocommerce_admin_order_data_after_shipping_address', 'custom_checkout_field_display_admin_order_meta', 10, 1 );