The normal position taken by the regulator is
“We do not normally count the storage provided within water butts or rainfall harvesting measures towards the attenuation storage requirements as there is no guarantee that these devices would be empty at the time that a rainfall event occurs”.
This is a reasonable position to take for standard rainwater harvesting design as it takes no account of the relationship between the demand and supply of the storage unit. For a complete explanation of designing rainwater harvesting systems for surface water control reference can be made to BS8515 Rainwater harvesting code of practice (2009, rev 2013). For a more in depth explanation the report by HR Wallingford “SR736_Developing Stormwater Management using Rainwater Harvesting” (2012) can be studied.
The principle which allows rainwater systems to be designed to provide surface water control (prevent runoff) is based on demand being greater (on average over a period of time) than the supply to it. There is uncertainty associated with the calculations for both the demand and supply elements and these issues are discussed in the HR Wallingford report.
However this rule (demand / supply ratio) is not seen as needing to be complied with if rainwater harvesting is only being used for meeting Interception requirements. A simple check to see that demand is greater than 5mm of rainfall runoff in 2 or 3 days would suffice, on the basis that rainfall only occurs on average about once every three days.
It should be noted that recent trends towards leakage flows from water butts and rainwater tanks to create storage in the system to attenuate the subsequent runoff does not comply with the principle of Interception even though they may have value in providing some degree of stormwater runoff attenuation.