..With the advent of the ISO 26262 there is an increased emphasis on top-down design in the automotive industry. While the standard delivers an ideal framework for safety design, it lacks detailed requirements for the various stages themselves. The lack of structure becomes evident in the reuse of legacy components and subsystems, a common scenario in the cost-sensitive automotive domain. This poses particular challenges in the context of automated driving where multiple subsystems both new and legacy need to coordinate to realize a function, leaving vehicle architects and safety designers to rely on experience for their decisions. This paper introduces a method to support consistent design of the Functional Safety Concept(FSC) as required by the ISO 26262. The method arises from and addresses need within the industry for systematic architectural analysis and rationale management and reuse of legacy subsystems. It reuses an existing work product viz. the diagnostic specification of a subsystem as an input and allows for the systematic assessment of the removal of human influence in the design of the subsystem. A report with an abstraction level suitable for a vehicle architect is generated and used as a basis for decisions related to the FSC. The proposed method is further tested in a safety critical braking subsystem at one of the largest heavy vehicle manufacturers in Sweden. The results demonstrate the benefits of the method including (i)systematic reuse of pre-existing work products, (ii)the collection of requirements for automated driving functions, (iii) the parallelization of work across the organization on the basis of expertise (iv) the applicability of this method across various types of subsystems.