In many public buildings, you need a EURO key to use the sanitary facilities. This key allows you to use accessible toilets and is valid throughout Germany and some other European countries. It costs EUR20 and can be ordered from the CBF association in Darmstadt (information in German).
In other European countries, the key can be ordered from various organisations (see the Wikipedia overview). In the United Kingdom, there is a comparable Radar Key. The CBF association offers a directory of barrier-free sanitary facilities in Germany, Austria and Switzerland (currently available in print only; a digital version is planned), while the Changing Places website offers similar information for the (United Kingdom). The accessalloo app provides details of barrier-free sanitary facilities globally.