You can plant a tree as close to a water line as you wish. However, it is important to remember that trees need water and should be watered periodically, especially during times of drought. If you do not have access to the water line or are not able to water the tree regularly, then it is best to plant the tree further away from the water line.

You should contact your local municipality to find out where the water line is located in your area. Many municipalities have regulations about how close trees can be planted to water lines.