First, I believe software blueprints should be put together by people with 15 to 30 years of expertise. Secondly, blueprints for a custom home are completely indispensable and are referred to regularly during construction. I believe one of the reasons why is because they are visual, not textual. I believe software requirements should be the same and then they would be easier to adjust and would retain their value.
Custom homes can be adjusted in mid-build. But it is a big deal and you have to get the architect involved and potentially have your plans revetted by inspectors and the city. It's a process that has a high cost. And it should because good blueprints like good software designs are highly dependent internally. I believe every change risks degrading their quality.
If you are in a project where there are large numbers of unknowns about the functionality of the project, I highly recommend shrinking the scope to what you know and actually understand. Build that, and then learn from it. Then come back around and start a new "dave's custom home waterfall" (for lack of a better name) process to implement the next set of functionality.
Constantly course correcting is a recipe for never finishing and weakening quality. There should be natural impediments to course correction and scope reduction should be used until the unknowns have been reduced to a reasonable risk level.