Classic single sinks, with or without a drainboard, usually have to be installed in a cabinet measuring 50cm or more in width.
Make way for a 20 inch sink
If you already have a kitchen unit measuring 50cm or more, you must make sure that any sink you buy matches the space available. No matter what size of sink you want, the size of the kitchen unit and the sink must line up. The selection of 500mm sinks available from SCHOCK can be found here.