Belsize Avenue is a street in the Belsize Park area of Hampstead. Located in the London Borough of Camden, it runs from Haverstock Hill westwards to a junction with Belsize Terrace and Belsize Park Gardens. It is the former carriageway approach to Belsize House, a country estate that occupied the area until it was demolished in 1853 to allow residential development. A number of houses in the street were built by William Willett around 1873.