"The architects are expanding two sites."
Translation:Die Architekten erweitern zwei Orte.
IMO you can't "Orte erweitern" or "Lagen" for that matter. That's why I took it that "sites" meant construction sites.
Hence my solution: "Die Architekten erweitern zwei Baustellen." which is understandable German.
"Grundstück" (from "Grund"="ground" and "Stück"="piece") just refers to a piece of land, whether or not something is built on it.
So, while a Grundstück may be extended, that would be a job for municipal administrators, not architects. If architects are the ones doing the extending, translating "site" as "Grundstück" doesn't make any sense.
Duo has plenty of other nonsense sentences, how are we supposed to know that this one is a special case (and also have knowledge of what particular construction jobs are the duties of municipal administrators)?