Basically, this depends on what is the main piece of information in your sentence.
If you want to say that you have a flat (i.e. you’re not renting it), then you should use «є».
If you want to say that your flat is in Kiev (i.e. not in Lviv), or that your flat is small (and not big), or that you have a flat (and not a detached house), then you don't use «є».
I believe this English sentence can be translated either with «є» or without it, both variants are possible. If one of them is not accepted, please use the report button.