The work involved in preparing a flood risk assessment can vary enormously between sites and can involve any or all of:
 
• Historical research into flooding on the site
• Topographical surveying of the site, the surroundings, and adjacent reaches of any watercourses
• Hydraulic modelling of storm events through adjacent watercourses and over the site and its surroundings
• Consideration of the effects of the proposed development upon surface water flows across and around the site
• Consideration of the effects of storm water drainage from the impermeable surfaces (roofs and pavings) generated by the proposed development
• Gathering information on existing drainage provisions from Local Authorities, Environment Agency, Statutory Authorities, Internal Drainage Boards, and Service Providers
• Carrying out infiltration tests on the site and considering the effects of using soakaways to dispose of storm water runoff
• Designing storm water disposal systems including soakaways, retention ponds, swales, attenuation and storage structures, infiltration blankets, permeable pavements and the like
• Consideration of the effects of climate change
• Consideration of overland flood flow routes and the provision of flood defences

The complexity of the assessment has more to do with the location, history, and topography of the site than the size or type of the proposed development. Thus a single block of flats on a small brownfield site in a flood zone in the middle of a town may present much greater challenges than a 10-acre mixed development on a greenfield site on the edge of a city.