Assessing the risk from legionnaires disease is a legal requirement. If you own /manage a building with water services in which the public/employees have access to then yes, you need a legionella risk assessment. This also includes rental properties (see extract from HSG 274 part 2)
"Carrying out a legionella risk assessment and ensuring it remains up to date is required under health and safety law and is a key duty when managing the risk of exposure to legionella bacteria".
The process involves the risk assessor walking through the water system, taking photos of assets and any areas of concern. He/she will then produce a report based on the water system and the current control measures in place. The report will include a risk rating system which will prioritise any action points.
Typically schematic drawings are also produced (if not already in place).
The report should be easy to understand and meet the requirements of BS 8580-1:2019 (please see link at the bottom of this page).
Brunel water services pride ourselves on giving clear and concise advise without elaborate and misleading jargon which many companies use to bulk out the report.