Straight staircase for single-family house: Are the dimensions okay?

Ergo, with a depth of 27 cm, the height may "only" be 15 cm. [...] Conclusion: I believe this convenience formula is too strict for the "average Joe"

I would even call that an uncomfortable step dimension. That is for princesses who want to majestically descend the stairs in ball gowns with long trains - in my opinion, a rather rare application in everyday residential use.