Submitting journal version of an article while conference version is still in review In my field (electrical engineering) it is a commonly-accepted practice for publications to be produced at various stages of the "evolution" of a research project, meaning that preliminary results can be published in a workshop paper, intermediate results in a conference paper on the same topic, and "final" or "definitive" results in a journal article. This paradigm is supported by the IEEE, for instance. As long as you cite the original work(s), state that the work is an expanded version of an earlier work (published at the lower stage of the "evolution") and obtain copyright clearances, this is a generally accepted practice. 

This leads to a significant reuse of content across generations. I have heard multiple times that the threshold for jumping from a conference to journal paper is "30-40% new material".  

My question is this-- do you need to wait for a conference paper to be accepted/published to submit the journal version of an article?  If you do not wait, does this violate the "multiple submissions" principle, even if the submissions are not identical, and not at the same "hierarchy". If so, why does waiting for the conference paper to be accepted make this "okay"? 

The reason for this question is the case where you are on a tight timetable to submit a journal article, but you want to include some conference results that are currently in review (and may be for several more months).
As a general rule, as long as you are honest with the editor and reviewers, you're OK:


Make it clear in your cover letter that you are submitting an extended version of a paper that is currently under review at a conference. Also clarify this anywhere else in the submission process where you are asked whether this submission is an extended version of an existing publication.
Submit a copy of the conference version together with your journal manuscript, so that reviewers and editors can evaluate the new contribution. Note on this copy that it is currently under review.


Some journals have a policy specifically covering this case. For example, IEEE Transactions on Wireless Communications: 


  It is also possible to submit the journal and conference version at the same time.  Or the journal version can even be submitted first. The journal version should not have appeared, however, at the time of submission of a conference version of that paper.