The way the bill is written it applies only to federal property, so only as long as a lighthouse is held by the federal government can this act apply to it.
One of the provisions of the act is that should the government want to sell a designated heritage lighthouse, then there must be some kind of protection put on that lighthouse as it goes out of the federal inventory. That could mean that as the government sells it we would seek provincial designation on it or we'd place conditions on the sale, so the new owner would have to look after the heritage character of the building. In some jurisdictions it's possible to get a heritage covenant or an easement on the property that requires the owner to maintain the heritage character.
The way the bill is designed, it's not that the bill applies once something is sold, but that as you sell it the new owner agrees to take on the obligation of looking after the heritage character of that building.