It runs from an interchange with the A 1 at Bargteheide to Wankendorf, halfway between Bad Segeberg and Kiel. A northbound extension to Kiel and a southbound extension connecting to the A 39 are being planned. The A 21 will cross the extension of the A 20 near Bad Segeberg.
In 2026, the A21 will be completed until Klein Barkau.
The A21 is planned to end at the Barkau Interchange when it's completed.