More easily, you can stand on the "inside" and build upwards and outwards. In order to cover a canal-facing brick façade with plaster, stucco, or some other ornament, scaffolding external can also be mounted much like the city's wooden docks are.

Historic Venetian construction processes are unique, making Venetian architecture take a life of its own largely independently from the otherwise very influential Tuscan-Roman architectural practices that impacted the architectural styles of the rest of the peninsula. Is there any specific aspect of traditional venetian construction you're interested in?