Custom Code Remediation: The Budget Line Everyone Underestimates

That assessment is where the budget surprises tend to originate. It is common for the initial complexity estimate for a piece of custom code to understate the actual remediation effort once a project team gets into the object in detail — discovering, partway through, that the object interacts with three other custom programs nobody had mapped, or that the “simple” report is actually load-bearing for a regulatory filing. Each of those discoveries becomes a change request, and change requests get billed at system-integrator day rates that, as covered elsewhere in this series, are already climbing as the 2027 deadline approaches.

The pattern compounds with the over-customization problem identified in ISG's 2026 research: more than half of the large-company decision-makers ISG surveyed admitted their organization had customized its legacy ERP to a degree that now makes standardizing risky. That combination — heavy historical customization plus underestimated remediation effort — is close to a textbook recipe for the budget overruns and schedule slips documented throughout the broader research on S/4HANA migrations.