This study presents a synthetic approach to the study of architectural form and function of annexes to early Christian basilicas on the Greek mainland and Crete (4th – 6th centuries AD). The introductory section is in two parts: the first deals with the state of the research on sacristies (described as diaconica in Greek literature) and baptisteries. The second part presents the liturgical and ecclesiastical sources from the early Christian period that illuminate (or obscure) the liturgical functions of certain parts of the church. The main part of the study comprises a catalogue of the monuments, which are registered in the five different dioceses of the Helladic area. Finally, the third part presents summarized conclusions, among which is the existence of eight individual types of annexes: 1) Baptisteries, 2) Sacristies, 3) Chapels, 4) Episcopal complexes, 5) Porches, 6) Rooms with domestic/agricultural function, 7) Towers/Staircases, 8) Funerary annexes.