You're not supposed to ask. It's like a structural engineer asking if it's okay to spend time doing a geological survey; it's not optional. Or a CFO asking if it's okay to pay down high interest debt. If you're the 'engineer', you decide the extent it's necessary
What if you spend a week or month refactoring something that needs a quick fix now and is being deleted in 1-2 years? That's waste, and if you went rogue, it's your fault. Besides, you always create added QA burden with large refactoring (yes even if you have tests), and you should not do that without a discussion first--even if you're the founder.
Communicate with your manager and (if they agree) VP if needed, and do the right thing at the right time.
Sure, if you're not sure if it's the right thing to do, talk to your manager or TL. A good engineering manager can help. If your manager "would never allow" it, they're not a good manager. Even for jobs much more menial than engineering, a good manager recognizes that autonomy/trust are critical for satisfaction and growth.
If you're working someplace where you're "not allowed" to make the changes you "wish you could," you're doing yourself a disservice. Find someplace where you're not only "allowed," but expected to have (or develop) the judgement required to make these decisions.
To be clear: "the business" expects (and in the medium/long term requires) engineers to make these decisions themselves. That is the job
The correct solution would of course rather be "you were the manager". :-(
The estimate was building in the time to get it done without breaking too much other stuff. For emergency things, Scotty would be dealing with that after the emergency.
If your captain is always requiring everything be done as an emergency with no recovery time, you've got bigger problems.
If I touch code that I am not supposed to touch or that does not relate to my direct task I will have HR talks. I'd be lucky to not get laid off for working on things that do not relate to my current task.
Structural engineers also don't commonly change structural features after initial delivery; realistically, I would expect changing a two-lane bridge to a four-lane bridge to be more expensive than constructing a four-lane bridge where none exists.