Having experienced multiple 're-build' projects in my career, I know that it is hard to capture all of the requirements from the working software. Often because of the size of the code base and lack of documentation. To the client, it seems clear what is required - that is all of the features from their existing project with an upgrade / change of framework and infrastructure.<p>In each case, either an estimate has been performed and failed to capture all of the required features, or the estimating process has been deemed too expensive to do thoroughly. And so the business (questionably) decides to use Agile / Scrum methodologies to complete the project.<p>This in effect means we have waterfall-esqe project in that it has fixed requirements, but an acceptance that we're going to have to work it out as we go along, in an imperfect 'Agile' manner.<p>This leads to a high probability of either project time / budget overruns or an unhappy client who perceives that the new software lacks capability that was present in the previous version.<p>Specifically, the question is: How do you estimate (in terms of time and cost) for a 're-build' where you are rebuilding an existing piece of software, and can agile be used?<p>I'm anticipating the following answers, but would like to hear from anyone who has experience of a process that has worked.<p>- Is the answer simply that you don't? Maybe the business has to accept that software that has evolved over years will be expensive to replace. How does a business make a decision whether the project is worth undertaking?<p>- Is the answer that the business has to agree to undertake the project in an agile manner, and monitor progress regularly to help them to decide whether to continue?<p>- Is the answer that the estimate should be undertaken and paid for as a project in it's self. You would then be tasked with estimating how long you need to produce a good estimate? Understanding a project properly could take an amount of effort not far short of implementing in code.<p>Related reading I've done before posting this question:<p>Is it possible to use Agile methodologies in a fixed price contract?<p><a href="https://pm.stackexchange.com/questions/29/is-it-possible-to-use-agile-methodologies-in-a-fixed-price-contract/45#45?newreg=f5fdb61bab6748af86fa8c9a63bec495" rel="nofollow">https://pm.stackexchange.com/questions/29/is-it-possible-to-...</a><p>In Agile project management, how do we estimate the completion date to win a contract?<p><a href="https://pm.stackexchange.com/questions/28051/in-agile-project-management-how-do-we-estimate-the-completion-date-to-win-a-con" rel="nofollow">https://pm.stackexchange.com/questions/28051/in-agile-projec...</a><p>Martin Fowler / ThoughtWorks on Agile estimation in general<p><a href="https://info.thoughtworks.com/rs/thoughtworks2/images/twebook-perspectives-estimation_1.pdf" rel="nofollow">https://info.thoughtworks.com/rs/thoughtworks2/images/tweboo...</a>