A underfloor heating system requires, according to DIN 18560, a covering of the pipes with screed in a thickness of at least 35-45mm for construction type "A" (pipes on thermal insulation/impact sound insulation), depending on the type of screed (calcium sulfate screed or flowing screed or cement screed). In this area, you can safely screw in a screw anchor or reduce the screed thickness by about 5mm as a safety margin when drilling. You can find the screed construction in the building description of your [GU] or the [LV] for screed work from your architect and you should be familiar with it.