What's the most common name -in the UK- for the owner of the apartment (in which rents it to others)?
I found three choices: homeowner, landlord, owner. Is one of them is more common in usage than the other?
Landlord is the correct term when the apartment is rented to others.
This can be via a letting agency or a direct relationship between both parties.
The landlord lets the apartment to the tenant.
The tenant rents the apartment from the landlord.
The landlord is not always the owner of the apartment. Sometimes a council (local authority) or university will lease apartments from the owners on a long-term lease and then let them to tenants for shorter periods.