Aufstockungen is the German term for vertical additions. These are rooftop additions common throughout European cities - where many structures were built with concrete, block, or stone. Vertical additions offer a really interesting path towards re-compacting (densifying) existing neighborhoods in an incredibly sustainable manner. They preserve more affordable, existing housing. They reduce sprawl. They allow the incorporation of new housing without sealing new surfaces - thereby reducing the urban heat island effect, and allowing more area for mitigating storm inundations and flooding. It is also an approach that can be utilized to add to a number of different building types - not just housing, but schools, offices, institutions, etc. Further reading... Aufstockungen: Innovative Density,   Mike Eliason's 2014 piece on vertical additions, via the Urbanist. Viel ungenutztes Potenzial: Dachgeschoßwohnungen am Gemeindebau (Untapped potential: Attic apartments in municipal buildings), Der Standard article on the potential for vertical additions and attic housing in Vienna's municipal housing. Wohnraumpotenziale in urbanen Lagen: Aufstockung und Umnutzungvon Nichtwohngebäuden, (pdf) TU Darmstadt study on the potential for new housing via vertical additions and office conversions, in the cores of German cities Sauerbruch + Hutton's mass timber addition to an existing DDR Plattenbau, for the Berlin Metropolitan school, via Baunetz. AO Architekten's mass timber addition to the HTL in Graz, Austria, via Detail. Lastly, to stay up to date with what Michael Eliason is doing at Larch Lab, be sure to sign up for newsletter updates.