Let's say that you have an old Cathedral and you want to build a spire on top of the cathedral and thus expanding it. I don't think renovating is the word, because it doesn't carry the meaning that it's going to be expanded through construction. It only implies that some parts of it will be repaired or remodeled.
In the case of residential property, we often talk about "adding on" a room, or "extending" an existing room. In the case of adding on, the new room is often called an "addition" to the building. In the case of institutional buildings, additions of multiple rooms with connecting hallways are often called "annexes" to the original building, but you wouldn't use this for a single room or a nonfunctional space like a spire.
In your example, we could just say you want to "add" a spire to the building.