First of all, the good news: you can relax because the Luxembourgers are giving you more time this year. Instead of hurrying up until March 31, 2021 as usual, cross-border commuters to Luxembourg can submit the tax return 3 months later until June 30, 2021.
This also applies if you are married and want to switch from an individual assessment (tax class 1) to a joint assessment (% rate on your income tax card).
For 2020, there are additional questions due to Corona:
1 I worked from home office in Germany. Do I have to pay tax in Germany?
A distinction must be made as follows:
If you only worked in the German home office because of Corona and would otherwise have worked in Luxembourg, you do not have to pay tax on this home office time in Germany. Rather, it is taxed in Luxembourg. The employer should provide you with a confirmation that you only worked from home due to Corona.
Conversely, if you had already worked from home or from outside of Luxembourg before Corona, the home office days / days of your stay outside of Luxembourg are usually not corona-related. Therefore, you typically have to pay tax on these days in Germany if you have exceeded the 19 day limit.
You should have the documents supporting the reasons for working from the home office. This is valid for a period of max. 14 years. This is the period a tax office can request information and tax retrospectively.
2 Can I use the new German home office allowance of EUR 5/month?
This is usually only possible if either
You have to pay tax on the home office days in Germany or
You have other income that is taxable in Germany and you have to calculate the income from Luxembourg employment to determine the applicable German tax rate.
3 What about payments due to short-time work (chômage partiel)?
The Luxembourg payments due to short-time work (chômage partiel) are also taxed in Luxembourg. So, if you have no other income, no need to file a German tax return.
4 What about special leave for family reasons / Congé pour raisons familiales?
As a rule of thumb, these days are also taxed in Luxembourg.
However, if you receive additional income that is taxable in Germany, this can lead to an increase in German tax.
5 What is the deadline for filing a tax return in Germany?
The German tax office has not yet made any statements about a possible extension. Thus, you should be prepared to submit the German tax return by July 31, 2021 if you have no tax advisor. In case you have a tax advisor, you are allowed to submit until February 28, 2022.