When arriving from a foreign country to settle in Luxembourg, the formalities to be completed in order to move in are not the same depending on the country of origin (from a member state of the European Union or an assimilated country, or from a third country).

EU or assimilated country nationals 

EU or assimilated country nationals and their family members (regardless of nationality) must register their arrival with their commune of residence if they intend to stay there for more than 3 months.

The declarant must present himself to the commune of arrival with the following documents:
- Valid identity card or passport (accompanied, if necessary, by the required visa or a residence permit issued by another EU Member State);
- if applicable, a family record book or, failing that :
o marriage certificate or partnership certificate;
o children's birth certificates.

Following the declaration of arrival at the municipality, the declarant may request a residence certificate for himself and his family members. Such a certificate justifies, among other things, the granting of extraordinary leave for relocation of an employee.

Within 90 days of his arrival in Luxembourg, the declarant must also fill in the following form at the same local administration
- a registration statement if he/she is an EU or similar national,
- an application for a residence card as a family member of an EU citizen if he/she is a family member of the EU citizen but is a third-country national.
 

Third country nationals

Third-country nationals who are not family members of an EU (or similar) national must declare their arrival to their municipality of residence within 3 days of their entry, regardless of the duration of the intended stay.

In the case of a stay of less than three months, they are exempt from this declaration if they have filled in an accommodation form with an accommodation establishment (e.g. stay in a hotel).

Third-country nationals staying in Luxembourg for less than 3 months must declare their arrival to the commune, presenting :
- a valid travel document, if necessary accompanied by a visa;
- if applicable, a family record book or, failing that :
o marriage certificate or partnership certificate;
o children's birth certificates.

Third-country nationals staying in Luxembourg for more than three months must present themselves to the commune of arrival with the following documents
- the original residence permit obtained before entering the territory;
- a valid passport, accompanied, if necessary, by the required visa or a valid residence permit issued by another Member State of the Schengen Area;
- if applicable, a family record book or, failing that :
o marriage certificate or partnership certificate;
o children's birth certificates.

Within 90 days of arrival, a third-country national with a temporary residence permit who wishes to stay in Luxembourg for more than 90 days must apply for the residence permit corresponding to the category of his or her residence permit (employee, self-employed, trainee, etc.) at the Immigration Department of the Ministry of Foreign Affairs.

Additional documents may be required by the commune. It is therefore advisable to check with the relevant local authority in advance.
If the documents are not in German, French or English, a certified translation by a sworn translator must be attached.

Source: guichet.public.lu