[ascar]

I don't see how that has anything to do with the rent control regulation? What would your expectation be without that regulation?

Doppelte Haushaltsführung is a tax relief for people needing a 2nd place close to work. Also the work of your gf needs to be at least 1 hour away from the main apartment [1], she has to pay at least 10% of the cost of the main apartment to be allowed to deduct it (and maybe more requirements that I'm forgetting right now). She also has to pay 15% additional Zweitwohnungssteuer on the rent of the 2nd apartment.

[1] https://www.haufe.de/personal/entgelt/verwaltungsregeln-zur-...