Answer
Missing documentation makes a takeover harder, but not automatically impossible. Part of the knowledge can be reconstructed from the source code, database, version history, configuration, logs and the behaviour of the application. Conversations with users matter even more: they often know the actual rules and exceptions that are not clearly named in the code.
The first task is not to write a complete manual retrospectively. First, the operating path, critical functions, data flows and known risks are documented. For particularly important processes, tests are written before larger changes are made. At the same time we clarify which documentation must be maintained in future, such as architecture decisions, deployment, interfaces and recovery. A flat fixed-price takeover without this exploration would be risky, because nobody can seriously know which hidden dependencies lie in the system.
As of 3 September 2026
Further reading
Related topic: Taking over existing software