Add checkout VAT and SSN fields

gistfile1.php

// Add VAT and SSN fields to the checkout form
add_filter( 'woocommerce_checkout_fields' , 'custom_add_vat_ssn_fields' );
function custom_add_vat_ssn_fields( $fields ) {
	if ( ! isset( $fields['billing']['billing_vat'] ) ) {
    	$fields['billing']['billing_vat'] = array(
        	'label'       => __( 'VAT', 'your-domain' ),
            'placeholder' => _x( 'VAT', 'placeholder', 'your-domain' ),
            'required'    => true, //change to false if you do not need this field to be required
            'class'       => array( 'form-row-first' )
        );
    }

    if ( ! isset( $fields['billing']['billing_ssn'] ) ) {
    	$fields['billing']['billing_ssn'] = array(
        	'label'       => __( 'SSN', 'your-domain' ),
            'placeholder' => _x( 'SSN', 'placeholder', 'your-domain' ),
            'required'    => true, //change to false if you do not need this field to be required
            'class'       => array( 'form-row-last' )
        );
    }

    return $fields;
}