When Lilacs Last in the Doorway Bloomed, is an old poem and implies they were close to the house. Mine is smack up against the corner of my foundation and it's such an airy plant that it has done no damage in 35 years. Of course you should always "Dig Safe" no matter where or what you plant in your yard and avoid digging anywhere you'd be bumping into utility pipes. You will love your lilac!

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.