The visit to the terraces of the Duomo of Milan can be done independently of the cathedral itself. For tickets to access the terraces, you have to go behind the cathedral, to a store (I think it was called Point Duomo) that is close to the National Palace. The ticket costs 8€ if you take the lift, and 5€ if you want to climb the stairs. We went by elevator, but keep in mind that there are times when there is a long line. We were lucky and in a little over 5 minutes we were going up to the terraces. Although part of the roof is still under construction, the architectural view of the Duomo up close is great.