When estimating (story points) a story that consists on extending a current functionality with a known tech-debt, should we consider the effort that will be spent to refactor the current code or should we estimate independently of this tech-debt?
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I would estimate without considering the technical debt. Of course this debt is still there and you probably have some indication of how much it is, so you should lower your velocity for that sprint.
That also shows to the product owner how much technical debt really costs: 'hey, normally we do 30 story points, but because of all this debt we can do only 20. How about some extra time to clean-up this mess so we can get a higher velocity later?"
I'd vote for estimating with the tech debt. The refactoring won't be free, so to neglect that cost is misleading to yourself and your customer.