If it is built correctly, it is possible to build a sunroom onto an existing deck. However, most of the time we find that the existing deck requires an upgrade to bring it in compliance for a sunroom addition. We prefer to remove the existing deck and replace it altogether. When our team rebuilds the deck as well as the sunroom addition, we can warranty our work from the ground up. If we build a sunroom addition onto an existing deck, we cannot warranty the deck and sunroom foundation structure.