Unable to answer "What’s the surface here?" for https://osm.org/way/241518129 via StreetComplete 43.2:

Construction site but changing, here are some images, what's "complete" so far.  Hopefully the images can guide you, on what it will be when complete, should find the building plans

Attached photo(s):
https://westnordost.de/p/94127.jpg
https://westnordost.de/p/94128.jpg
https://westnordost.de/p/94129.jpg
https://westnordost.de/p/94130.jpg
https://westnordost.de/p/94131.jpg
https://westnordost.de/p/94132.jpg