Explaining to Children Why They Cannot Have a Dog

I have the same problem right now because my son wants a second dog. We have a noisy dachshund who we love very much and my son has asked if we could get a companion for him. I made a promise that as soon as a second dog can be accommodated we will go out and get one. We can't get a second dog now because it would be a riot but because my son wants ne, we are moving into one of the houses that we are renting out so we can accommodate a second dog.