Hi,
Addresses can be really complicated. The full guide is on the wiki: https://wiki.openstreetmap.org/wiki/Addresses
Personally, I would open up JOSM, select all the buildings and slap addr:street
and addr:housenumber=123
on them. However, I would add addr:unit=XYZ
on every one of them (as @Krzyzwen@feddit.de pointed out as well).
Next question: does every single house have their own letterbox? Or is there one location where all the letterboxes are located? Add a https://wiki.openstreetmap.org/wiki/Tag:amenity%3Dletter_box with addr:street
and addr:housenumber
as well!
I don't think that adding an address to the plot of land is very useful (but a name of the project/area might be nice as well). Have a look at https://wiki.openstreetmap.org/wiki/Key:place for this, place=plot
or place=city_block
might be appropriate depending on the situation.