German mortgage banks based on stock, more frequently founded from the early 1860s, used the traditional Pfandbrief system to cope with the growing tasks of urban and housing construction. Its safety for both creditors as well as debtors of real estate financing depended not least on a clear limitation of business activities. During the building boom of the late 19 th century, however, many mortgage banks undertook a massive expansion of their business and developed highly complex corporate structures, so that the well-established rules for lending on real estate were increasingly undermined. Using the examples of the “Pommern-” and the “Preußenbankgruppe”, the article explores the distortions resulting from the building boom of the late 19 th century and analyses the economic, penal, and legislative means that were used to solve them. The study concludes that, due to their severity, the crisis phenomena in the real estate market of the 1890s led to a uniform regulation of mortgage banks across the Reich for the first time, which indeed achieved a sustainable stabilization of the Pfandbrief system: the price, however, was the isolation of German mortgage banking from developments in Western Europe.