Berlin Regenwasseragentur — Sponge-City Rainwater Management Rules for New Development

Berlin's combined sewer system used to discharge into waterways one to two dozen times a year in heavy rain, which is considered unsustainable. Annual combined-sewer overflow volumes reported by the water utility to the Berlin parliament ranged from about 2,293 thousand m³ (2016) to 7,280 thousand m³ (2017) and 3,947 thousand m³ (2021).
The Regenwasseragentur was set up in 2018 by the state and Berliner Wasserbetriebe to advise administrations, housing companies and owners. Since 2018 rainwater in new urban development must be infiltrated, evaporated or otherwise managed on the plot, and discharge to the sewer is approved only in rare justified exceptions. A newer building code requires green roofs on new buildings with roofs up to 10° pitch and from 100 m², and green-roof and facade funding for existing buildings was improved.
The agency says the sponge-city layout of the Buckower Felder housing area is cheaper than a conventional sewer, but that is its own claim rather than an independent evaluation. Unsealing existing surfaces is described as the weak point with little progress so far, and BUND Berlin criticises the effort as small relative to the problem; the water utility itself says decoupling most sealed areas will take decades and large sums. A 1998 decision targets roughly 300,000 m³ of underground combined-sewage storage in inner districts by 2025.

Read the full analysis: https://www.energiezukunft.eu/klimakrise/berlin-auf-dem-weg-zur-schwammstadt