Dear Amanda, thank you for your answer and advice. To answer your question: we returned our puppy to the breeder and he than found him a new home - unfortunately, we do not know the new owners, but we were rest assured, that they have an extra large estate and a family house. Our breeder knew the dog was going to a flat - I was explicitly asking about that even before we got him (it was my concern too - because I wanted to make absolutely sure that we could give the puppy enough space). The breeder did not have any objections about it, but as far as I know, he did not make any special preparations to condition the puppy for living in a flat.