| Lilacs get huge! I planted mine eight feet away from the house. It is pushing it there. It a white one and has gotten ten feet tall and six feet across. They can get huger if you let them. I would be less worried about pipes than the branches scratching against your house, I would never ever plant one up against the foundation. If you want a lilac and a lilac smell by your doorway, plant a smaller cultivar of syringa (if there are any?), I think there is something called "Korean lilac" or "persian lilac" (don't know latin name) that I see in nurseries that is a small bush which smells really nice. I like the large old-fashioned lilacs better, but just make sure you leave enough room. | 
