(1) In the case of a lump sum charge for operating costs, the lessor is entitled to apportion increases in operating costs proportionately to the lessee by making a declaration in text form, to the extent that this has been agreed in the lease agreement. The declaration is only effective if the basis of the apportionment is referred to and explained in it.
(2) The lessee owes the part of the apportionment allocated to them from the beginning of the second month following the month in which the declaration is made. To the extent that the declaration is based on the fact that operating costs have risen with retroactive effect, the declaration has a retroactive effect from the date on which the operating costs rose, but no earlier than the beginning of the calendar year preceding the year of the declaration, provided the lessor makes the declaration within three months after they first had knowledge of the increase.
(3) If operating costs are reduced, then the lump sum for operating costs is to be reduced accordingly from the date of such reduction. The lessee is to be informed of the reduction without undue delay.
(4) If advance payments of operating costs have been agreed, then, after a settlement of accounts, each of the contractual parties may undertake an adjustment to a reasonable amount by a declaration in text form.
(5) In the case of changes in operating costs, the principle of economic efficiency is to be observed.
(6) A deviating agreement to the disadvantage of the lessee is ineffective.