Attic not fully registered as living space - NRW

I quote from the Beck commentary: "If the agreed use of the rental property cannot be exercised for building code reasons, there is a defect...(OLG Nürnberg ZMR 1999,255) If it turns out during the tenancy that the rooms rented as living spaces are subject, among other things, to a building regulation usage restriction due to insufficient ceiling height, the rent shall be reduced from the time of knowledge of the restriction, even if the rooms were accepted and lived in as contractually agreed. (LG Lüneburg WuM 1989, 368). A prerequisite for assuming a material defect (lack or impaired suitability for use) is that the competent authority prohibits the use or that official intervention can be expected in this respect. (OLG Düsseldorf ZMR 2000, 707) Thus, the tenant cannot invoke a defect as long as the competent building authority has not objected to the insufficient ceiling height in the rooms. (LG Berlin GE 1989,91)"

Why should the tenant be allowed to reduce the rent?

Because the actual rental area differs from that stated in the contract.

She is referring to the fact that the sqm specification in the lease does not correspond to the sqm of the apartment.

And what if no area is specified in the lease?

sqm is a mandatory disclosure