In fact, what you are doing is tying the developer's hands and making it less possible for them to nimbly work around unforeseen obstacles or repurpose existing solutions without raising a "process exception" to reopen the spec or sizing of a work item.
Be clear about the goals but let the developer(s) who have their hands in the code make the decisions about how to implement it. That means you can't really roadmap a big project down to the minute, but those roadmaps were always lies so there is literally nothing lost except for some fantasy Gantt charts.