"It is a kitchen."
Translation:Es ist eine Küche.
As a fixed phrase, when you say "it is..." then you say "es ist..." and that applies to everything, regardless of gender. An exception to that rule is for living things, so e.g. you could say "Er ist ein Junge", but at birth for example, exclaiming the gender of the newborn baby, you would definitely say "Es (das Baby) ist ein Junge/Mädchen!". That would also work for a dog or so, as in "Is your dog male or female?" - "Sie ist ein Weibchen".
I haven't been able to find a definitive answer on this either. I would have thought either would be correct. It must just be one of those things you have to learn. Sort of like in English you can refer to a ship as a she. I guess it's just not natural to personify a kitchen. Any native speakers care to chime in?
According to my grammar book and the way I was taught in class, third-person pronouns substitute for nouns, whether persons or things. The example given is --Woher hast du den Ring? --Ich habe ihn in Ulm gekauft. Er war gar nicht so teuer. Maybe using es can (but not necessarily must) be a special case when gender isn't known beforehand, but I haven't found anything to explain why sie must be incorrect.