Is it okay to explore other grad school options while deferring admission for other reasons? So the school that I got into for m PhD program offers a simple admission deferral process that I'd like to apply for, for personal reasons (I don't want to use this post to discuss whether or not I should defer; assume that the personal reasons are serious enough, because they are). This could defer my admission one year, to fall 2019. 

My question is, would it be very unprofessional/unethical to also apply to other schools during my deferral period? It seems silly to throw away my offer, since I would like to take it, but I would also like to see where else I may be able to get in. Would it be better to simply rescind my acceptance, rather than having the department believe that I am certain on my intentions to enroll in a year?

Edit to clarify: The decision to defer is due to the personal reasons that I mentioned above, and not because I take issue with the program to which I've been admitted. If not for the personal reasons, I would go to school now. But, if I'm going to be deferring at all anyway, it would seem imprudent to not at least try my hand and open up the future by applying elsewhere. Telling the department about the personal issues and leaving out the previous sentence, though, does seem wrong.
I think it is at minimum rude, and probably unethical.

In deferring, you have still accepted a position. If you use the remaining time to do things other than what you said you were going to do, you are taking advantage of the generosity of the offered deferral (which, I would add, is not a guarantee everywhere - they are generous to offer it).

I can see a couple possible exceptions:


If the reason for applying to another school is also related to a serious personal issue (for example: to be near an ill family member, to be near an institution responsible for your own care, because a certain climate is important to your health, for immigration/travel reasons).
If there is no offer of funding/support from the school, then in my opinion you are less beholden to them, and they are less impacted by if you are to later withdraw your acceptance. In this case you should almost certainly pursue other offers.
If the deferral process really is that simple, where the reason you give doesn't really matter: they are just allowing you to take a year for any reason. I think this sort of thing is more common in undergraduate rather than graduate admissions (especially for a PhD), though I don't have enough personal experience to say. I would still suggest talking to someone at the school before deciding this is the case.


I would also add that I think you could be vastly overestimating the importance of attending a particular school and paying far too much attention to ranking. While I was writing this answer, @JonCuster posted what I think is a very important comment to consider, coming from the perspective of someone who has spent a lot of time here at Academia.SE:


  ..."I see too many questions here about changing to get into a ‘better’ school, with little thought given to what makes it ‘better’ - some random ranking does not make it better. You appear to have thought through it a bit more, so keep thinking!"


If you do want to look at other options because you aren't certain about attending this school, I would suggest discussing this with them rather than rescinding your acceptance outright. If I were them, however, I would have much preferred to admit a student this year that actually wants to go to this school rather than waiting a year to take someone who doesn't.