In the past I have often wondered if I have provided enough architecture for the software team to work to. Conversely I always like to leave enough room to maneuver for the guys to do design for their own parts of the software, I would really hate it if I was working on a project where everything was defined and all I needed to do was "code in the blanks".
The idealist in me wants to provide just enough architecture to start work at all, and emergency design will do the rest. I have found this is not how great software is made. When everyone is left somewhat to their own devices, the code starts to lose coherence. Everyone will code their own mini framework that will not interact well with other stories or allow extension for all requirements. This is natural as everyone might not be aware of all requirements for the project.
The answer to this is pretty simple in practice, whenever two or more stories meet the following criteria you need some architecture:
1. Other code interacts with the product of a story.
For example some kind of remote or local service or data. In my current work we do a lot of inter-process communication. This is really a technical detail, web services or in-process framework meets the same criteria. Whatever it is, it needs some documented architecture. The documentation part here is important, it should be simple to find look-up the exposed interfaces with arguments including valid values, errors etc. This rule tends to reduce errors and confusion (more errors) occurring from not knowing about or not understanding .
2. Two or more stories are similar
For example, two kinds of message or input that could be handled in the same way or are similar enough to be categorised together. Polymorphism and/or inversion of control can be used to have a central mechanism handle these in a reusable way. This rule tends to reduce the overall amount of code and improve code quality.
These two scenarios are sum up pretty much of every time where I had wished I had provided some architecture before work commenced