"The house has a garden."
Translation:Υπάρχει ένας κήπος στο σπίτι.
I'll give this a try but we'll need corroboration from a moderator. I think if one uses έχω then it's ok to drop the indefinite article, as in έχω πονοκέφαλο, "I have a headache," but if you want to emphasize how bad the headache is, then the article can indicate it, έχω ένα πονοκέφαλο, "I've got a [bad] headache." With υπάρχει, in colloquial speech the indefinite article is probably normally used in this case and to omit it might sound katharevousa because ancient Gk didn't have an indefinite article, which is to say that spoken demotic (everyday Gk) nowadays uses the indefinite article in more situations. Source: Mackridge, The Modern Gk Language (Oxford, 1985) 6.1.2 (pp. 198ff). He writes (202): "In journalistic and official writing an effort is often made, under the influence of katharevousa, to omit the indefinite article in contexts in which it would appear in ordinary speech." However, a lot has changed since the 1980s, so maybe in 2019 that is no longer the case.