First, you need to choose if you are going to improve it, or ultimately abandon it. That's not an easy decision. If you decide to abandon it, you need some very

First, you need to choose if you are going to improve it, or ultimately abandon it. That's not an easy decision. If you decide to abandon it, you need some very convincing arguments, and lots of them. Before making that decision, work with the codebase, at least until you understand it well enough. It will allow you to uncover arguments either way. Improving a codebase is a well-known subject, so I'm not going to comment on it further. If you decide to ultimately abandon it, you need to understand it won't happen tomorrow, and perhaps not before a few years (for example, I'm 2 years in with a codebase I decided to abandon, and it's probably going to be at least 1 more year in production). Stakeholders hate when you spend time just rewriting it for the sake of it (from their perspective). What you want to do instead is use a strangler pattern: your new codebase should "strangle" the old one, and deliver value VERY quickly, which will convince the stakeholders it was the right choice. First, all new features are in the new codebase. If possible, start with easy features that have as few dependencies as possible with old codebase. Any call between the two should be in a special wrapper in your new codebase, so you can start having a sense for what code will need to be rewritten, at some point. Then, start to "strangle" the old codebase: wrap ALL calls so they go through the new codebase first. That will allow you, somewhere in the future, to cut off the old codebase part by part, and avoid the full rewrite effect, as well as quickly revert to old codebase if bugs are uncovered. Once you have that, you can more easily identify which parts should be replaced first: performance issues, too many bugs, new features needed... When you reach the end, it's a matter of convincing stakeholders you absolutely need to go to the last mile, with good arguments. If you can't find good arguments, you probably don't need to go to the last mile yet.