I work with utility companies on multi year migration projects to move their entire tech stack. This includes, a different database, different data schema acros

I work with utility companies on multi year migration projects to move their entire tech stack. This includes, a different database, different data schema across thousands of tables, new front end components, desktop and mobile applications, multiple third party integrations and the training and documentation for the whole thing. The utilities are heavily regulated, often beholden to taxpayers, lawmakers and public utility boards and working on use it or lose it budgets with hard cutoff times for delivery and go-live dates. It's not just budgets and time constraints that make it impossible to do flexible estimates. Their internal personnel all have their regular work duties to attend to while supporting the migration and they can't drop everything for years at a time to dedicate support for the project. Add in the coordination with multiple vendors and you need to be hitting your time estimates, planned years in advance, within a week. Sometimes budgets don't matter that much, once they are a year into a three year project, they will find the money if it's needed, but the coordination alone requires this kind of accuracy. It's amazing what necessity does to your estimates. We have become really good at it. This includes things like "Oh we're working with Oracle, add 3 weeks to that integration just because" or "this is a mobile app for the service techs who tend to be resistant to change, add another week for training and a month for revisions". Yes this is just "padding" but it's very specific padding that's tailored to the type of work being done and has so far been accurate and continually getting better for us. edit: I should add that our estimation process involves multiple week long workshops with all parties. We go over every tool, process, integration and technology currently in use and then write a detailed design document that goes through 2 rounds of review with the customer before being signed off on. These design documents become the basis for a secondary contract to do the actual work and the customer understands that if it's not in the document, it's not getting built or migrated. Any additions require a contract change order.