I drive 40 kilometers a day to my first place of work (there and back). However, the tax office only accepts the one-way distance, i.e. 20 kilometers. Why?

The tax office applies a commuting allowance for travel between the home and the first place of work. This amounts to €0.30 for each full kilometer of travel between the home and the place of work. For the calendar year 2021, the distance allowance will increase to €0.35 from the 21st kilometer. From 2022, the flat-rate distance allowance will be €0.38 from the 21st kilometer. The actual number of kilometers driven is not taken into account.
The distance allowance is generally limited to €4,500 per calendar year. This does not apply if you use your own car or a car provided for your use to travel between your home and place of work.