note= key is intended to help inform other mappers about something, such as help disambiguate some complicated map situation, it's not really intended to encode information.

I encountered such situations a couple of times, but in OSM there's no tag to explicitly record "start of construction". Luckily active constructions last 2-3 years so such a tag won't be relevant for too long.

You could tag this whole thing as building under construction, and then update its shape as construction progresses and satellite imagery gets updated. Or add floors and units info once the building is done.