(1) The contractual parties may agree in writing that the rent is to be determined by means of the price index for the cost of living of all private households in Germany computed by the Federal Statistics Office (Statistisches Bundesamt) (indexed rent).
(2) While an indexed rent is applicable, the rent, except for increases under sections 559 to 560, must remain unchanged for at least one year in each case. An increase under section 559 may only be demanded to the extent that the lessor has carried out structural measures due to circumstances for which they are not responsible. An increase under section 558 is excluded.
(3) A change in rent under subsection (1) must be asserted by declaration in text form. In this declaration, the change in the price index that has occurred as well as the rent in the individual case or the increase are to be indicated as an amount of money. The revised rent is to be paid at the commencement of the second month beginning after receipt of the declaration.
(4) Sections 556d to 556g are to be applied only to the initial rent of an agreement providing for an indexed rent
(5) A deviating agreement to the disadvantage of the lessee is ineffective.