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Form 100 :

The Tax Administration asks you to send in your return by March 31.

If you send it after 31 March, it should in principle be accepted.

However, if you file late (i.e. after March 31) or do not file, it may take longer to process your tax return.

Non-resident taxpayers, married before January 1 of the tax year, who wish to change their tax class are obliged to send their return before March 31, otherwise they will automatically be taxed in tax class 1.

Form 163 :

The deadline is 31 December and in principle the Tax Administration does not accept forms after that date.
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