If you are an EU citizen or family of an EU citizen, then you are legally considered to be residing in whichever member state you are physically in. This is true for short periods (a weekend visit) or for long periods (a lifetime).
EU citizens have unconditional right to reside in any member state for periods of under three months (see Article 6 below). As long as you are self sufficient, working or studying, you also have a right to reside in a member state for a longer period of time (see Article 7 below). Family of EU citizens have the same rights, but must be accompanying their EU citizen.
The core requirements for “residence” (for EU citizens and their families) comes from Directive 2004/38/EC:
Directive 2004/38/EC
CHAPTER III
RIGHT OF RESIDENCE
Article 6 – Right of residence for up to three months
1. Union citizens shall have the right of residence on the territory of another Member State for a period of up to three months without any conditions or any formalities other than the requirement to hold a valid identity card or passport.
2. The provisions of paragraph 1 shall also apply to family members in possession of a valid passport who are not nationals of a Member State, accompanying or joining the Union citizen.
Article 7 – Right of residence for more than three months
1. All Union citizens shall have the right of residence on the territory of another Member State for a period of longer than three months if they:
(a) are workers or self-employed persons in the host Member State; or
(b) …[are self sufficient]
(c) … [are a student]
(d) are family members accompanying or joining a Union citizen who satisfies the conditions referred to in points (a), (b) or (c).
As this Directive has been transposed into the law of each European member state, so to have these requirements for residency.
The UK transposition of Directive 2004/38/EC has the corresponding language in describing UK requirements for residency in the UK:
The Immigration (European Economic Area) Regulations 2006
PART 2 – EEA RIGHTS
Initial right of residence
13.—(1) An EEA national is entitled to reside in the United Kingdom for a period not exceeding three months beginning on the date on which he is admitted to the United Kingdom provided that he holds a valid national identity card or passport issued by an EEA State.
(2) A family member of an EEA national residing in the United Kingdom under paragraph (1) who is not himself an EEA national is entitled to reside in the United Kingdom provided that he holds a valid passport.
The UK rules for issuing EEA family permits (section 21.4.1 – Handling and assessing applications for EEA family permits) are also explicit that the family member of an EU citizen is “resident” if they are lawfully in an EU country:
The non-EEA national could equally have entered the country in some other category (visitor, student etc) and would still be considered as lawfully resident in that Member State. For example, an Indian national married to a French national, who had obtained a visa to enter France as either the spouse of the French national or in some other category (as a visitor, student or work permit holder etc), would be considered ‘lawfully resident’ in France, if, at the time of application, they were abiding fully by the conditions of that visa.
The Irish transposition of Directive 2004/38/EC has the corresponding language in describing Irish requirements for residency in Ireland:
S.I. No. 656 of 2006
European Communities (Free Movement of Persons) (No. 2) Regulations 2006
Residence in the State
6. (1) Subject to Regulation 20, a person to whom these Regulations apply may reside in the State for up to 3 months on condition that he or she -
(a) (i) where the person is a Union citizen, holds a valid national identity card or passport,
(ii) where the person is not a Union citizen, holds a valid passport, and
(b) does not become an unreasonable burden on the social welfare system of the State.
I have resided in three member states since April 2006: Germany, Italy, France. In each case, I was there for a few days.
For people covered by this Directive (EU citizens and their families), there is no legal requirement that you have to stay for a long time (e.g. more than 6 months) in order to legally “reside” in a member state. This Directive (and it’s local transposition) are the primary EU law regarding residence of EU citizens and their families in other EU member states.