Newly built basement smells musty

That's how it will be. And the smell lingers for a long time because a basement is hard to ventilate. Drying devices wouldn't be a bad idea. Karsten

Ventilating for me means opening the windows fully about 3 times a day for approximately 15-20 minutes and then closing them again, and being very careful with tilting them. We also set up heaters in the basement for drying, but the masonry can certainly still contain residual moisture... Yesterday a friend who has built three houses himself was with us – he found the smell completely normal for the basement in the screed/concrete/cement state. He was thus familiar with the smell from his houses, no cases of mold. Of course... 100% certainty requires an inspection/measurement by a professional. Thank you very much for your many responses!!! Regards Franz

In summer, it is rather difficult to remove moisture from the basement because the absolute humidity in warm air is significantly higher. Dry airing works best when it is freezing outside.

In a house, many thousands of liters of water are stored. That takes its time. Concrete chemically cures and then only dries slowly. So don't worry, especially if your acquaintance is already familiar with it.