In the event of a divorce or a legal or de facto separation, the marital status at the end of the tax year, specifically on December 31, is decisive for the tax assessment. Pursuant to Art. 42 para. 2 of the Federal Law on Direct Federal Taxation (DBG), a separate assessment is made for the entire tax year if the spouses are legally divorced or actually or legally separated at this time, whereby each spouse is obliged to prepare an individual tax return for the entire assessment period. The cantonal tax laws, such as § 56 para. 2 StG/LU of the Tax Act of the Canton of Lucerne, are generally based on the federal requirements. A de facto separation is legally assumed if at least one of the spouses has decided to give up the marital partnership, the joint household has been dissolved and there is no longer any financial community with regard to housing and maintenance.
The date of the final divorce decree, the separation decree or the de facto dissolution of the marital partnership is decisive for the initiation of the separate tax assessment. In the absence of a written agreement immediately after the separation, a plausible explanation of the separation situation is sufficient for the tax authorities. Formal proof of separation is not required peremptorily. In accordance with the tax equality of registered partnerships with marriage, there are identical tax consequences for registered partners in the event of separation or dissolution of the registered partnership.