Leave your dog for several short periods in the day, maximum 5-10 minutes. How long will depend on how your dog reacts. Ensure your dog has shelter and feels warm, safe and comfortable when you leave them.
When you return, don’t acknowledge the dog as soon as you walk in the door. Walk in, pretend to put your shopping on the bench or take your coat off before paying attention to your dog. Ignoring your dog when you leave or return conveys the message that you coming and going is nothing to get excited about. Do this several times until your dog is comfortable with you leaving home. Gradually increase the times to one hour, then two, then the afternoon and so on. Don’t push it or you could go backwards.
It takes time, which is why you need to start conditioning well before you leave them on their own, so it gets used to being left alone. If possible, ask a neighbour to listen for any barking, whining or howling and to time how long this goes on for. Monitor this over time to see if it lessens or increases.