Hello, bentek!
Talk to the owners of 111a and 113a. Together, the rear area can be developed - no one can do it alone without building encumbrances or significant restrictions.
You can bring up the topic of building encumbrance, but you will probably not get approval because your own development on 111a or 113a would almost be finished with it, which means nothing else than that the building encumbrance would be very expensive. If I were the owner of 113a, I would roughly squeeze €6K per year (!) from you for the building encumbrance (estimate depending on the property value). In my opinion, you can never earn that additionally with your property, or your property would be dead because of it.
So, land consolidation and reallocation, otherwise in my opinion it won’t work. If you get both neighbors on board, you can also approach the owner of 109, then it gets really good because you can share the development costs and the developable area can be used much better.
Best regards
Dirk Grafe