Cases in which a joint declaration can be filed with the spouse and children, if they live outside Spain
First of all, it is necessary to determine in which state you reside, in principle if you reside in the Member States of the European Union, and if you could exercise this option in other countries.
Personal Income Tax has been configured following the Constitutional Court Ruling 45/1989, of 20 February, as a tax in which the general rule is individual taxation. Joint taxation is an optional regime, consisting of the accumulation of income obtained by individuals who are members of a family unit, defined in the terms established in Article 82 of Law 35/2006, of 28 November, on Personal Income Tax and partially amending the laws on Corporate Tax, Non-Resident Income Tax and Wealth Tax (BOE of 29 November), which provides
"1. Persons who are part of any of the following forms of family unit may be taxed jointly
1.ª The one integrated by the spouses not legally separated and, if they exist:
(a) Children who are minors, except those who, with the consent of the parents, live independently of the parents
(b) Children of full age who are legally incapable of exercising their parental authority and who have been extended or rehabilitated.
In cases of legal separation, or when there is no marriage, the one formed by the father or mother and all the children who live with one or the other and who meet the requirements referred to in rule 1 of this article.
No one may form part of two family units at the same time.
Therefore, joint taxation consists of an optional form of declaration of different taxpayers of the Tax, who accumulate their income, presenting a single declaration.
Consequently, a joint return cannot be filed by persons who are not taxpayers for this tax, and taxpayers are individuals who are resident in Spain for tax purposes in accordance with the provisions of Articles 8, 9 and 10 of the Tax Law.
Family resident in European Union Member States
This is regardless of the fact that, due to the obligations derived from the Treaty on the Functioning of the European Union and the Agreement on the European Economic Area, a deduction has been established that applies to family units formed by taxpayers of this tax and by tax residents in Member States of the European Union or the European Economic Area, established in the forty-eighth additional provision of the Tax Law, and a declaration option for taxpayers resident in other European Union Member States established in Article 46 of the revised text of the Non-Resident Income Tax Law, approved by Royal Legislative Decree 5/2004, of 5 March (BOE of 12 March).
Family resident in other States
However, this deduction and option, as it derives from the application of Community law, would not apply to family units whose members are not resident in countries of the European Union or the European Economic Area.