The planning instincts that work cleanly on a new project quietly break on an existing codebase. An experienced lead who can deliver a greenfield application on time and on budget can land inside a brownfield engagement and miss the estimate by fifty percent — not because they got lazy, but because the assumptions they were relying on do not carry over. This piece walks through the assumptions that need to invert when the project changes type, and how to estimate and plan honestly for each case. The assumption that fails first: discovery cost On a greenfield project, discovery is mostly about the business. What does the user need? What does the data shape look like? What are the regulatory constraints? Once those are answered, the engineering team has clear canvas. On a brownfield project, there is a second layer of discovery that does not exist in greenfield work: mapping what the existing system actually does. Not what the documentation says it does — what it does.…