With the information you provided we can give you the following advice:
We don’t know whether your residence title according to § 26 of the Residence Act contains a residence requirement at all. You would have to take a look at it. If you have one, you will have to apply at the responsible foreigner’s authority if you want to move to your fiancé.
However, because you are a Hartz IV recipient you must – in any event – submit an application with the Job Center when moving from Berlin to Siegen. Whilst this is possible in general, there are a few things to bear in mind:
No community of needs
Since your fiancé works and earns income, whereas you are currently living on Hartz IV, you must declare to the Job center that you and your fiancé do not form a so-called community of needs to stay eligible for these unemployment benefits. As partners who move in together, you must only provide support for each other financially if you also manage your affairs together. That is, you have a joint account, and you can dispose of your fiancé’s income or if you have joint contracts (insurance, building society contracts, etc.).
Therefore, you would have to make sure that you separate the finances cleanly from each other. According to a ruling by the Essen Regional Social Court (Case No.: L19 AS70/08), simply living together is not sufficient to oblige the earning partner to also bear the costs for the other, i.e., to assume a community of needs.
Community of needs on trial
A young couple must first be given the opportunity to test whether they want to stand by each other in times of need and life’s vicissitudes. This can take up to a year. The legal definition of a marriage-like community offers unmarried couples the opportunity to first move in together on a trial basis, even with Hartz IV. Accordingly, the partners can initially live together for a year and receive the higher Hartz IV rate for single persons before they will be classified by the job center as a community of needs. Such an arrangement is colloquially referred to as a community of needs on trial. There is no separate legal definition of this. From a legal point of view, it is nothing other than a shared apartment.
Hence, you should tell the job center in Berlin that you are planning to move in with your fiancé in Siegen to see if you are a good fit in day-to-day life (probationary period). Subsequently, you have to register at the job center in Siegen after you have changed your registration address.
Good luck and have a nice time in Siegen!
You can find more advice from Angelika e.g. here
This article was published in German.