(1) If the lessor has carried out modernisation measures within the meaning of section 555b no. 1, 3, 4, 5 or 6, then they may increase the annual rent by eight per cent of the costs spent on the dwelling. In the case governed by section 555b no. 4a, the increase is permissible only if the lessee is able freely to select the provider of publicly accessible telecommunications services via the service connection installed and the lessor does not apportion or has not apportioned as operating costs a provision fee under section 72 of the Telecommunications Act (Telekommunikationsgesetz).
(2) Costs which would have been necessary for structural maintenance measures do not belong among the costs expended in accordance with subsection (1); where necessary, they are to be ascertained by way of an estimate.
(3) If modernisation measures are carried out for more than one dwelling, the costs are to be apportioned reasonably among the individual dwellings.
(3a) If increases of the annual rent as defined in subsection (1) are effected, then, leaving aside the increases in accordance with section 558 or section 560, the monthly rent may not increase, over the course of six years, by more than three euros per square meter of residential space. Where the monthly rent amounts, prior to the rent increase, is less than seven euros per square meter of residential space, it may not increase, in derogation from sentence 1, by more than two euros per square meter of residential space.
(4) The rent increase is excluded where, also taking account of the likely future operating costs for the lessee, it would constitute a hardship that is not justifiable even taking the legitimate interests of the lessor into account. The interests are not weighed against each other in accordance with sentence 1 if
1. the property merely was restored to a customary condition, or
2. the modernisation measure was carried out as a result of circumstances for which the lessor was not responsible.
(5) Circumstances that constitute a hardship in accordance with subsection (4) sentence 1 are to be taken into account only if they have been notified in good time in accordance with section 555d (3) to (5). The provisions on the cut-off period defined in sentence 1 are not to be applied if the de facto rent increase exceeds that which had been announced by more than 10 per cent.
(6) A deviating agreement to the disadvantage of the lessee is ineffective.