The chapter outlines the main steps and necessary tools within the methodological-operational approach for the digitisation of the cognitive and conservation process of historic districts. In particular, this chapter explicitly presents the tools and methods underlying the digital code of practice developed for the technical knowledge of the historical architectural built heritage, as well as for the semi-automatic derivation of intervention classes and priorities by a Technical Decision Support System (T-DSS). The code, called 3hD-DigitalCode, is developed through the structuring of a logical-relational scheme for the qualification of the built environment by digital models based on the CityGML standard, as well as digital systems exploiting Virtual Reality techniques. However, a modular structure has been conceived to support a multidisciplinary information system, maintaining the geometric models of the buildings as the central core, reflecting the correct dimension of the digitised architectural heritage (from building to district). In fact, all the thematic modules determined for the in-depth study of specific knowledge domains are related to the geometric core, thereby supporting a coherent structure of technical knowledge.