Set the direction, then govern the delivery.

Architecture frameworks exist to fix the order in which decisions are taken. The TOGAF Architecture Development Method works through an architecture vision, then business architecture, then information systems and technology architecture, before it reaches opportunities and solutions, then migration planning. The order carries the logic: a migration plan written before the target state is defined is a procurement schedule rather than a transformation plan. A buyer should establish which architecture decisions are already treated as fixed and what evidence is expected to close the rest.

Public sector buyers are usually working to the HM Treasury Five Case Model, under which a business case has to hold on strategic, economic, commercial, financial and management grounds together. The economic case rests on appraising options against a business as usual baseline, so a paper offering one option has not been appraised at all. Benefits are estimated at approval and only known after go-live, and knowing them depends on the current position having been measured before anything changed. Without that baseline a post-implementation review has nothing to compare.