Code Reviews in general (not specifically at Code Review Stack Exchange) will often review the design. The people doing the review also have influence on the design too, so it is natural.

#Off-Topic

The design itself though, is a very broad, and unsubstantial concept. On Code Review (and Stack Exchange in general), it would be too broad to ask:

> I have this design in mind to implement FeatureABC. Is it a good design? Can it be improved?

The reason it is too broad is because ideas are intangible, there are too many right answers, and too many opinions, possibilities, and considerations.

So, on the one hand, pure design reviews are off-topic on Code Review, even though they are often reviewed in real-life code reviews.

This particular meta question asks, though: *When, and why, is "reviewing design" on-topic on Code Review and when is it off-topic?*

- **It is off-topic when the only thing wanted/demanded in the review is a 'design review'.**

Further, the actual implemented code is heavily dependent on the actual design. You cannot change the design without heavily impacting the code. It follows that:

 - if the design changes, the code changes
 - if the design is not 'fixed', then the code is not 'fixed'.

Now, if a question provides a design description, and matching code, then requests: *This is my design, is the design OK?*, it will be **off topic**. This is because:

 - if the design is still to be reviewed, it is logical that the design is not stable.
 - if the design is not stable, then the code is not stable.
 - additionally, if the design is not stable, then the code is off-topic because one of the on-topic questions is: ***To the best of my knowledge, does the code work?***
 - If you don't know the design, then you don't know what the code is supposed to do.
 - If you don't know what the code is supposed to do, how can you possibly know that it works?
 - the code is not ready for review if the design is not agreed.

Thus:

- **Any question requesting a design review is off-topic, either because the code is not there, or because, even if there is code, the code is just an example of what one particular design would/could look like. If the design changes, then the code is irrelevant**

So, any question requesting a design review is off-topic.

#On-Topic

So, when is reviewing the design on-topic?

When there is real code (which implies a real & stable design), and the code is sub-optimal **because** of the design!

If the code is having to do awkward, inefficient, or otherwise broken things to make the design work, then it would be on topic to say:

> This code would be better if you could restructure the design and do it *this* way instead.

- **design reviews are only on-topic when the question does not ask for one!**

(yes, exactly, only when the design review is not requested, is the design review on-topic.... and that is why we are Code Review, and not Design Review).