Booze wrote: ↑Thu May 01, 2025 12:11 am
Hopefully someone can help me out here. The story as I recall is that this was simply a storage building before the Germans arrived. [...]
And knowing that this might sound stupid, what is the evidence for this building existing before the war?
 
According to 
Auschwitz: 1270 to the Present by Dwork and Van Pelt, the Auschwitz I camp began as a "migrant worker village" in or after 1910 (p.59). It was taken up by Polish troops in 1920 because of its ample housing space. The authors do not go in to any detail as to the construction of what they term the "ammunition depot" other than to say that it was "built by the Polish army in the 1920s" (p.322). Then they provide this German survey drawing of the site from December 1939 (p.325), which should be an accurate representation of how the Poles left it. The ammo depot is visible in the lower left.
Pressac's 
Auschwitz describes it as "The former powder magazine, or according to other sources, victualling store, of the Austro Hungarian and subsequently Polish barracks" (
p.129). He follows this up with a photo he took of a German handwritten list of numerous drawings of construction work to convert the building into a crematorium.
I think the evidence is satisfying enough, and I see no reason to question it. At the same time, it does not very much clarify what the building was before the German occupation. However, one of the listed drawings is titled as "Entwässerung [drainage] des Krematorium." This could correspond to the construction of the drain or drains in question.