From the 2016 income year onwards, you can usually amend your tax return yourself even if the deadline for submission has passed.
Check whether you can make changes yourself or whether you have to submit an appeal:
More about the topic
Appealing against property tax
You can only appeal to the Norwegian Tax Administration in cases where the municipalities use the Norwegian Tax Administration’s residential property values as a basis. Read more about appealing against property tax
Appealing against your tax assessment
If you want to appeal against your assessment, e.g. because the amount of withholding tax deducted or additional advance tax is wrong, you must send your appeal to the tax collector within three weeks after you receive notification of the decision. The appeal must state what you're appealing against and the basis for your appeal.
Errors concerning Young people’s housing savings scheme (BSU)
If the basis for young people's housing savings is wrong, you must ask your bank to notify the Norwegian Tax Administration of the correct amount, in addition to submitting an appeal against your tax assessment.