Add custom dimension fields to each variation settings for variable products

With the 2 hooked functions below you will get exactly what you are expecting like in your mock:

// Add variation custom "dimentions" fieldsadd_action( 'woocommerce_variation_options_dimensions','add_variation_options_built_dimensions', 10, 3 );function add_variation_options_built_dimensions( $loop, $variation_data, $variation ){    $variation_built_lenght = get_post_meta($variation->ID,"_built_lenght", true );    if( ! $variation_built_lenght ) $variation_built_lenght = "";    $variation_built_width = get_post_meta($variation->ID,"_built_width", true );    if( ! $variation_built_width ) $variation_built_width = "";    $variation_built_height = get_post_meta($variation->ID,"_built_height", true );    if( ! $variation_built_height ) $variation_built_height = "";    ?>    <p class="form-field form-row dimensions_field built_dimensions hide_if_variation_virtual form-row-last">        <label for="product_built_length"><?php            // translators: %s: dimension unit            printf(                __( 'Built dimensions (L&times;W&times;H) (%s)', 'woocommerce' ),                get_option( 'woocommerce_dimension_unit' )            );        ?></label>        <?php echo wc_help_tip( __( 'Built length x width x height in decimal form', 'woocommerce' ) ); ?>        <span class="wrap">            <input id="product_built_length" placeholder="<?php esc_attr_e( 'Built length', 'woocommerce' ); ?>" class="input-text wc_input_decimal" size="6" type="text" name="built_lenght_<?php echo $loop; ?>" value="<?php echo esc_attr( $variation_built_lenght ); ?>" />            <input placeholder="<?php esc_attr_e( 'Built width', 'woocommerce' ); ?>" class="input-text wc_input_decimal" size="6" type="text" name="built_width_<?php echo $loop; ?>" value="<?php echo esc_attr( $variation_built_width ); ?>" />            <input placeholder="<?php esc_attr_e( 'Built height', 'woocommerce' ); ?>" class="input-text wc_input_decimal last" size="6" type="text" name="built_height_<?php echo $loop; ?>" value="<?php echo esc_attr( $variation_built_height ); ?>" />        </span>    </p>    <?php}    //Save variation custom "dimentions" fieldsadd_action( 'woocommerce_save_product_variation','save_variation_options_built_dimensions', 10 ,2 );function save_variation_options_built_dimensions( $variation_id, $loop ){    $built_lenght = $_POST["built_lenght_$loop"];    if(!empty($built_lenght))        update_post_meta( $variation_id, '_built_lenght', sanitize_text_field($built_lenght) );    $built_width = $_POST["built_width_$loop"];    if(!empty($built_width))        update_post_meta( $variation_id, '_built_width', sanitize_text_field($built_width) );    $built_height = $_POST["built_height_$loop"];    if(!empty($built_height))        update_post_meta( $variation_id, '_built_height', sanitize_text_field($built_height) );}

Code goes in function.php file of your active child theme (or theme) or also in any plugin file.

This code is tested and works for WooCommerce 2.6.x and 3+.

You will get this: