My rental contract in the Netherlands forbids it, but at one time I discussed it with a lawyer and it's not an enforceable clause. Eventually I had to get the lock replaced and I have no intention of giving the property company a key.

>the apartment owner notices it, tries to enter your apartment to extinguish it, cannot enter because of changed lock, and the building burns down.

The fire department/emergency services would absolutely be allowed to (force) enter...why should the landlord be the savior? Perhaps it's different in Germany.