The authors agree that the main base of the surname is toponymic: it derives from place names, names of localities and of particular constructions —small and medium inhabited centres— formed from casa and its northern variant ca, together with their respective derivatives and compounds. Among the most relevant place names are Casèlla and Casèlle (in the Genoa, Turin, Milan, Bologna, Verona, Treviso, Salerno areas, among others), Casina and Casine (in the Pisa and Pistoia areas), Casone and Casoni, and Casaccia and Casazza (in the Sondrio, Ancona, Genoa and Bergamo areas).

E. De Felice notes that the base form Casa is very rare and peculiar to central and southern Italy, while Ca corresponds to the north. The types with Da, Dalla and Della are common in the north, especially in Liguria. The suffixed derivatives are frequent throughout Italy: Casini predominates in Tuscany, Casoni in Lombardy and Emilia-Romagna, the apocopated forms are peculiar to the northeast of Italy, and Casiello, Casillo and Casolla are typical of the Naples area. De Felice adds that, in minimal part, Casa, Casella and perhaps Casini can be, especially in southern Italy and in Tuscany, aphaeretic hypocoristics of Benincasa.

E. Rossoni specifies the areas of presence of the base form: the Vicenza area, Rome, the Latina area, the Naples area and the southwest coast of Sicily. Rossoni also records the variant Case, typical of the Belluno area —especially of Agordo, Gosaldo and Selvazzano Dentro—, which would likewise derive from names of localities with the root casa.

V. Blunda proposes, for the Sicilian variant, a different origin: the Greek casàs, which designated the cloth cover placed over the saddle or the back of pack animals. The surname would have arisen as a trade name —referring to the one who made those pieces— or as a nickname applied to the one who habitually used them in his work.