Also I've generally found it's easier to develop starting with a blank directory tree (no code), than to inherit a legacy codebase, have to make the sometimes grueling time/energy/focus/trial-and-error investment needed to come up to speed on it, understanding-wise, at the fine-grained level of detail you need to code confidently, then, figure out how to make a positive change that doesn't make some other thing worse. (And it's harder if no automated tests or documented manual test plan.) I call it OPC for Other People's Code. One of the anti-patterns of software engineering. It's distinct from NIH (Not Invented Here), which is another anti-pattern.
Of course it's always easier. The question is, can the business afford to wait while you greenfield another app? Usually it can't. That's why refactoring.
First step is fixing the development environment / build process and getting a staging server up. It will inevitably be broken / nonexistent, with frequent edits directly to production necessary. The last guy will have internalized a great deal of operational workarounds that you'll need to rediscover then codify into the app.
Next you write tests. There will be none. Once you have a workflow that is decent, you can start to identify the worst offenders. All the while, you'll be having to change the codebase to meet project requirements, this will give you a good idea of where the really bad shit is. Unit test all of it, and if you're feeling froggy, write some integration tests. Once you get to this phase, you should be unit testing your project work.
Only after those two are completed can you start refactoring. Treat it like TDD. Keep an eye on larger goals like 12 factor conformance. It may look pie-in-the sky at first, but it will give you ideas on what to focus on. Main advantage of refactoring over ground-up re-writing is, you don't have to sell it to your boss. You just do it, in and around normal project work.
The biggest hurdle is the first step. It's scary to fuck with deployment. The approach I've come up with is to fork the codebase and rebuild the tooling on top of that, deploying first to staging, then to production, alongside the current working copies. Once you're satisfied flip the switch. You may have to flip it back, but at least it will be easy and instantaneous.
These lessons are from my ongoing project to modernize an ancient Rails 2.3.5 website running on Debian Lenny. Linode doesn't even offer that OS anymore, I had to cannibalize a server with an EOL app on it for a staging environment. I can't use Vagrant because there aren't any Lenny boxes.
Do it anyway. The thing about testing is that it's a skill that you have to work on. You have to know what to test and how. Testing shouldn't affect the speed at which you write code at all.
The reason your boss is saying no is because you had to ask him. The reason you had to ask them is because you know it will take more time than it will save, at least at first.
You have to learn this skill somehow, and the best way to do it is on something that matters rather than with a side project. So write tests at your job, and learn the skill of testing on your employer's time, without their knowledge. Or on off hours if that makes you squeamish. But learn the skill, it's important.
Then, when you refine your testing work flow to the point where it makes more sense to test as you write, don't bother hiding it anymore. When they ask, show them your workflow and how it's not taking up too much time and list out the benefits of testing. If they tell you to stop anyway, take your new skills and find a new job. You're growing past the ability of your current job to challenge you.
