How to avoid ambiguity in "I am renting an apartment in New York"?

To "unequivocally communicate that I am the tenant (or the landlord)", among other things you could say one of the following.

• As tenant, I rent an apartment...
• As landlord, I rent an apartment...

As previously noted by choster, adding out to original sentence works for landlord sense:

I am renting out an apartment in New York.

You can state a not-landlord case, and perhaps imply tenancy, via

I pay for renting an apartment in New York.

As a renter I usually think of the tenant as renting, but the dictionaries seem to disagree with me as to primary use.

If you are the landlord, you could say

I am renting out an apartment in New York.

I have an apartment for rent in New York.

I am letting an apartment in New York.

Rent is indeed ambiguous that way.

However, the two senses have different syntactic affordances.

• I rented a house in Ypsilanti. (ambiguous: Subject = tenant or owner)
• I rented a house in Ypsilanti from him. (unambiguous: Subject = tenant)
• I rented a house in Ypsilanti to him. (unambiguous: Subject = owner)
• I rented him a house in Ypsilanti. (unambiguous: Subject = owner)
• I rented out a house in Ypsilanti. (unambiguous: Subject = owner)

And there's lots of other ways to disambiguate them; in context, one is rarely confused.

Tenant: I'm living in a rental apartment in New York.

Landlord: I'm renting out an apartment in New York.