A high-rise residential structure and or high-risk building is defined as having 7 floors or has a storey at least 18 metres high and having at least 2 residential units from the relevant relevant ground level.
Additional information can be found on the gov.uk 'Guidance on Criteria for higher risk buildings' web page.
To register a High Risk Building please visit the gov.uk 'Register a high-rise residential building' webpage.