IMO you should be spending time on refactoring and simplifying code without asking permission for it. Improving the code-base is part of the job and you should

IMO you should be spending time on refactoring and simplifying code without asking permission for it. Improving the code-base is part of the job and you should be setting aside about 20% of the time you estimate for new features to do so. There's no need to ask management for permission, your estimate is simply 20% larger.