If the site has been derelict for over five years, the appropriate consultant must assess the previous drainage network and make reasonable assumptions to establish probable flow rates and volumes. They should use best practice simulation modelling to determine the 1 year and 100 year peak flow rates at the relevant discharge points. A site visit prior to development will be required unless accurate data already exists from a previous survey. The resultant professional report can then be used to determine the pre-development volumes and rates of run-off. Without this professional input, the site must be deemed as greenfield pre-development, assuming Soil type 5 for the calculation of the pre-development site run-off.