Gross yield is a starting point
Gross yield divides a year of rent by the purchase price. It is useful for a first comparison, but it ignores most of the costs of owning.
What net yield includes
Service charges, maintenance, agency fees and periods without a tenant all reduce what you keep. Ask for the service-charge history of the building.
Compare like with like
Compare homes of the same size and age in the same community, and use recent leases rather than asking rents.
General information only, not legal or financial advice. Rules, fees and thresholds change, so confirm current requirements with the relevant authority or a qualified professional.