There are some interesting challenges with projects like this in London: "It's estimated that there's about 17,000 tonnes of bombs that were dropped on London

There are some interesting challenges with projects like this in London: "It's estimated that there's about 17,000 tonnes of bombs that were dropped on London in WWII and the rule of thumb is about 10% failed to explode," https://www.bbc.co.uk/news/magazine-23518137 https://www.bbc.co.uk/news/magazine-23518137 [I guess UXO must be an even bigger problem for German cities].