In Croatia, Kostanjšak families are Croats (from Pregrada area ). In the past century, relatively most of Croatian residents bearing this family name were born in Zagreb and Klanjec.
Prevalence
About 30 people with faimily name Kostanjšak live in Croatia today, in 12 households. There were 10 of them in the middle of the past century, and their number tripled. They are located in only threee Croatian places: in Zagreb (10), Razvor in Pregrada area (10), and in Opric (<5).
Outside Croatia
Family name Kostanjšak is present in this form only in Croatia.