This paper presents the ìGeislinger Konventionî as a widely accepted tool to optimize the operation costs of rental apartments in the Federal Republic of Germany. For a long period these operating costs have not been in the focus of the housing industry. The estimated amount of Ä 40.000.000.000, - annual costs for the 23.000.000 rental apartments show that it is worthwhile to put strong efforts into this area of Real Estate Management. The ìGeislinger Konventionî was developed by a group of scientist as well as practitioners of the real estate industry and major housing associations. It allows the management of rental apartments to start a benchmarking process. For a better understanding of this problem the paper will introduce in the complicated legal structure of operating costs on which the new developed tool is based on. The paper will show the result of a successful cooperation between the Real Estate Industry, housing associations and the scientific world in developing a new agreement for standards and evaluation for operating costs for Real Estate Management.