The landlord can only demand a supplementary payment for the heating and operating costs if he provides the tenant, no later than 12 months after the end of the accounting period, with a written statement of accounts proving that the advance payments for the operating costs were insufficient. In case the accounts result in a credit balance in favour of the tenant, this shall be paid out immediately. The landlord is not entitled to offset a credit balance against any claims that are disputed or have not been established as final and absolute. The tenant must inform the landlord no later than 12 months after receipt of the statement of accounts of any objections against the statement of accounts.