The debate on sustainable development has lead to the view of buildings as flows (mass, energy, money and information) or capitals. In this context buildings are considered as the largest physical, economical, social and cultural capital of a society. In Germany many institutions record different kind of data about buildings. Unfortunately there are just a few basic statistics about the amount of buildings. Collection of data is very complicated, often expensive and the handling of missing data is one of the biggest handicaps. With the exception of data about residential buildings and particularly monuments, it is an unsolved problem to determine the total number of buildings. Thus the main issue of this article is the description of an appropriate estimation procedure. This procedure relies on 12,430 communes and refers to data from the Cadaster of Real Estates and the Federal Office for Building and Regional Planning (BBR). The estimation is based on statistical data from well-known and easily accessible institutions. The number of buildings is estimated for communes with missing data. Using methods from the, so called, Urban Data Mining approach, unsuspected relationships are found in the urban data. ... mehrThese relationships are valuable for the estimation. The quality of the estimation is analyzed by training and test data sets. Information optimization leads to the conclusion that 20% of the communes hold 80% of all buildings. For an improvement of the estimation it is essential to refine the amount and quality of data in the larger communes. © Springer-Verlag Berlin Heidelberg 2010.