Dedicate 20% of your team’s schedule to “sustaining engineering.” This means allowing time for refactoring, fixing outstanding bugs, improving engineering processes, doing minor cleanup, and providing ongoing support. Take this into account in every planning session. Unfortunately, 20% is not enough to do big projects, so additional planning will be needed to get major technical rewrites or other big technical improvements. But without that 20% time, there will be negative consequences with missed delivery goals and unplanned and unpleasant cleanup work.
