Facts
The spouses [A.] and [B.] (appellants, taxpayers) had their tax domicile in [U.] /ZH and deregistered in [V.] /GR before the end of the 2011 tax period. In the canton of Zurich they remained economically liable to tax on account of real estate ownership.
The canton of Graubünden claimed personal liability to tax for the tax periods 2011 to 2014. The tax administration of the canton of Graubünden (KSTV/GR) issued the assessment decisions for direct federal tax and for cantonal and municipal taxes for the 2015 tax period on 2 March 2017. Both decisions became final without challenge.
The canton of Zurich already requested the taxpayers on 22 November 2016 to set out their centre of life. On 16 February 2018 the tax office of the canton of Zurich (KStA/ZH) issued the assessment decision for cantonal and municipal taxes for 2015; direct federal tax was not addressed. This decision was also not challenged.
On 26 March 2018 the taxpayers applied to the canton of Graubünden for revision of the 2017 assessment decisions. The KSTV/GR dismissed the applications on 5 February 2021: the classic grounds for revision were not present, the Graubünden tax act provided no ground for revision for intercantonal double taxation, and the taxpayers should have challenged the Zurich assessment.
The appeal to the Administrative Court of the canton of Graubünden dated 1 March 2021 was limited to cantonal tax; the objection concerning direct federal tax was stayed. The Administrative Court dismissed the appeal by judgment A 21 9 of 20 July 2021. It held the municipal tax to be final despite an incomplete statement of legal remedies, because the professionally qualified representation should have recognised the defect. On the merits it denied a ground for revision under Art. 141 para. 1 lit. a StA/GR, because the taxpayers had not challenged the Zurich assessment, the right of appeal was forfeited and no basis of legitimate expectation existed.
By submission of 2 November 2021 the taxpayers lodged an appeal (designated as a constitutional law appeal) with the Federal Supreme Court and requested that the decision be set aside. The lower court and the tax administration of Graubünden requested dismissal, the FTA waived a motion, and the tax office of Zurich requested dismissal.
Considerations
The appeal is treated as an appeal in public law matters (Art. 82 lit. a, Art. 86 para. 1 lit. d FSCA, Art. 73 DTHA). The Federal Supreme Court applies federal law of its own motion and reviews with full cognition (Art. 95 lit. a, Art. 106 para. 1 FSCA). For cantonal law that is harmonised, free cognition in principle applies; for purely cantonal law review is limited to arbitrariness (Art. 9 Cst.). The findings of fact of the lower court are binding (Art. 105 para. 1 FSCA).
A current intercantonal double taxation exists, since both cantons claim personal liability to tax for 2015 and the assessments are final (Art. 127 para. 3 Cst.). However, the time limit for appeal regarding intercantonal double taxation (Art. 100 para. 1 and 5 FSCA) has expired, because the taxpayers did not bring about a decision of final cantonal instance in the second assessing canton of Zurich. The application for revision in the first assessing canton cannot replace this time limit.
Revision under Art. 51 DTHA or Art. 141 para. 1 lit. a StA/GR presupposes discovered material facts or decisive evidence that already existed before the principal proceedings (genuine novas). The assessment decision of the second assessing canton may in principle come into consideration as such a fact. Under Art. 51 para. 2 DTHA or Art. 141 para. 2 StA/GR, however, revision is excluded if the ground for revision could have been raised in the ordinary proceedings with reasonable diligence.…