How To Only Allow 1 Variation of a Product Added To Cart

Add this PHP code to the end of your child themes functions file or custom functionality plugin to restrict a specific variable product so only 1 variation can be added to cart.

Swap out the 13 in the following PHP code with the product ID you want to restrict.

add_filter('woocommerce_add_to_cart_validation', 'wpsitesdotnet_restrict_single_variation_in_cart', 10, 5);
function wpsitesdotnet_restrict_single_variation_in_cart($passed, $product_id, $quantity, $variation_id = 0, $variations = []) {

    $specific_product_id = 13; 

    if ( $product_id == $specific_product_id ) {
    
        foreach (WC()->cart->get_cart() as $cart_item) {
        
            if ($cart_item['product_id'] == $specific_product_id && $cart_item['variation_id'] != $variation_id) {
            
                wc_add_notice(__('You can only add one variation of this product to the cart.', 'woocommerce'), 'error');
                
                return false;
            }
        }
    }

    return $passed;
}

When a customer tries to add more than 1 variation to cart, they get stopped and a notice displays this message “You can only add one variation of this product to the cart”.