[TheM00se]

The original engineer who came up with the code has laid out several key areas that need serious reworking within the documentation. The code base is a few years old and 30% of it is deprecated. In order to implement the new features, the code base will have to be refactored to better accommodate those features since the current code base has no such ability to accommodate such features.

[kohanz]

You're getting closer to a business case, but if you're confident in your assessment, you need to put it plainly, in terms that management can digest quickly. The bottom-line is money, and in engineering, time = money.

So here's the case: the time to add the new features on top of current implementation + the time to support this new, perhaps brittle, version = X. The time to refactor the application and add the new features + support the new, perhaps more robust, version = Y. You must show, in reasonable detail and with confidence, that X > Y.