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.
Since it's "I have", in principle the "є" is there.
Ordering by the strength of ownership:
"Я маю": in central Ukraine or in "textbook" Ukrainian, sounds really strong, almost like "I own this apartment". In the West is a normal way to say a neutral "I have".
"У мене є": Not so strong as "я маю", more natural in central Ukraine. Simply means you have an apartment.
"У мене": if you don't skip "є" it sounds like you have bought the apartment. Skipping it makes it more ambiguous, could be that you are just renting it (most likely, otherwise you wouldn't have skipped "є"...)
At least that's my native speaker attempt to explain how I perceive these three, it's not a rule I guess :')