It is impossible to give you a good answer with the information available. The age and method of construction also make it more difficult.
That is a large opening and it could possibly be a structural problem.
It would be best to have a local professional look at the structure and give you a recommendation. He might be familiar with how the building and chimney were originally constructed. To get an "unofficial" idea of the possibility, you could also check on what was done in similar flats.
In many areas, it is not common/permitted for a chimney to support the structure because of the shrinkage and deflection of a wood structure. If you have construction more permanent than wood, it could be a portion of the entire structure.
Last edited by concretemasonry; 10-30-2006 at 10:13 AM.