If the taxpayer discovers that the tax should be higher than the last known tax, he is obliged to file a supplementary tax return or a supplementary assessment by the end of the month following the month in which he discovered this and to pay the difference within the same period. This obligation shall continue as long as the period for assessment is running. The last known tax is the resulting tax as finally determined by the tax authority in the tax proceedings for that tax to date. The taxpayer shall be entitled to file a supplementary tax return or a supplementary assessment for a tax lower than the last known tax within the above-mentioned period if the tax has been finally determined at an incorrect amount. A supplementary tax return or a supplementary assessment for a tax lower than the last known tax shall not be admissible if any of the decisions on which the last known tax is based was made by means of an aid or on the basis of a tax arrangement. The taxable person shall be entitled to submit, within the same time limits set out above, a supplementary tax return or a supplementary assessment which does not change the last known tax but only the information previously claimed by the taxable person.

If a legal entity is dissolved with liquidation, the obligation to file a tax return continues until the date of dissolution of the legal entity. The legal entity shall be obliged to file a proper tax return within 30 days from the date of its entry into liquidation, for the part of the tax period that has elapsed before the date of its entry into liquidation. In addition, the legal person shall be obliged to submit a regular tax return within 15 days of the date of preparation of the proposal for the use of the liquidation balance, for the part of the tax period which has elapsed before the date of preparation of that proposal; this period may not be extended. If, after the date of the proposal for the use of the liquidation balance, a tax liability arises for that person, that liability shall be deemed to have arisen up to the date of the proposal for the use of the liquidation balance and the legal person shall be obliged to submit a supplementary tax return.

If the last part of the privatised property of a state enterprise is transferred to a body which is responsible for the management of the privatised property, the state enterprise shall be obliged to file a proper tax return within 30 days from the date of such transfer for the part of the tax period which has elapsed before the date of transfer.

In insolvency proceedings, the taxpayer is obliged to file a proper tax return for the part of the tax period that has elapsed up to the day preceding the effective date of the decision on insolvency and for which it has not yet been filed, no later than 30 days after the effective date of the decision on insolvency.