What possibilities you would have today to possibly still realize your project depend, among other things, on how the term "Kleinsiedlungsgebiet" is defined and interpreted by the building authority today. It is certainly no longer up-to-date, as it originates from the 1930s, when many homeowners still valued keeping small animals and gardening for their own needs on their property. Today, this hardly plays a role anymore – the plots are then too large. One should first research the initial situation with the authorities and then propose an appropriate usage/development concept. This would also include, for example, documenting the type and extent of the current development on site and, if necessary, using it as a comparison.
All of this should be done by an architect who also coordinates with the authorities and can discuss on an "equal footing" – or better – with the official representatives. Then many things are possible that are formally not initially permissible. Deviations in individual cases are often possible – well justified and appropriate.