Should I relate a house to its garden?

Hi Alex, welcome back to OSM editing. :-) It's not so common to map private residential gardens at all, at least not in the areas I usually map. Not common, but it does happen, so you are definitely not wrong doing it. I would say, though, that the postal address either belongs only to the building or to the complete plot. I would not tag both the building plus the garden with the address. I think you have three options:

1. leave the address with the building and remove it from the garden.
2. remove the address from both and add a single node somewhere on the property and add the address to that one.
3. similar to the last option but instead of a node, draw a line/area around the plot and move the address there.

Should you relate building and garden? You COULD add them both to a site relation and also add the address there but that feature is not well used (in my opinion) and will be hard to be maintained as many will just ignore those relations when making any changes.