I think there's a bias in the US against this sort of thing that doesn't exist (or not to the same extent) in Europe due to the age of the cities/buildings.
In the US, a building from the 1700s is a historic artifact to be cherished, while in parts of Europe a building from the 1500s is just the local pub.
So, the US is often hesitant to modify these old buildings, but Europe seems to have more of a perspective of "it's a building, not a museum, let's give it new life by modifying it."
This is just from the perspective of me, from the US --- and I think these old/new buildings are really neat!
Huh, thanks for that perspective, I (US native) was firmly in the "how could they do this?!" camp, but you're right! When old ass buildings are all over the place it's probably much more pressing to figure out how to allow ongoing development and construction, vs. how to preserve the aesthetic of yet another centuries-old brick structure.
Edit to add: ugh I still can't get over how ugly and deliberately discordant it looks, reasonable takes be damned!
Every country, even in Europe, is full of old but still disposable buildings of not much value.
In Italy we desperately want to hold onto every historical building like it's the peak of human achievement, but in the end that takes too much resources and you end up half-assing the job in most of them.
At least in Belgium those old/new buildings didn't have so much to do with allowing continuing construction, as much as expanding the function of some institution while keeping some continuity. The one in OP is a Port Authority (Antwerp is one of the major ports in the continent, after Rotterdam) and they needed to expand, so they kept the old building and added... A flying ship. The glass and steel thing is supposed to be a flying ship. The old building is below it, very nearly intact.
Another cool one for me was the Antwerpen-Centraal train station. They preserved 100% the look of the facade, and expanded in the back and below. It has train tracks on 3 different floors (none of which are the ground floor). Look at some of the pictures of the inside.
I was definitely including "expanding for extra use" in what I mean by allowing construction, but either way thanks for the info. I've seen some utterly hideous stylistic choices (which, to be fair, seem garish and "loud" by design, I didn't uncover some secret), but I really like when it's done with some care. Probably not a realistic goal in every case, as y'all have helped me see.
How do the floors in the upper structure handle the sloping incline of the geometric shape? Is there just a lot of closed off volumetric slivers between the planes of the floor and ceiling and shell, or is there only one or two floors, with the upper floor having a larger rising canopy?