Even though it is not completely necessary, it is advised that you carry out a Legionella Risk assessment every 12 months to keep the risk of Legionnaires’ Disease and other water hygiene hazards at a minimum. You should also consider carrying out a Legionella Risk Assessment immediately if the following changes occur: If there has been a reported outbreak of Legionnaires’ Disease in your area. If there has been a change of water system in your property/building. If the water system in your building has been moved to a different location. If the purpose of the location where your water system is based has changed.