you need to consider off-site flows in your design.

either
a) you manage those flows somehow
b) adjacent property owner manages those flows
c) or you get flooded

what happens in the interim when your site gets built, the adjacent offsite area does not get developed, flood hits and then your subject site gets flooded?  Eventually the offsite may be developed, but until then, subject site is at risk.

Offsite conditions must be considered relative to the impact of the onsite improvements.  For example if an existing flow channel is to be used, what are the impacts of the post development flows versus the existing condidtions?  If the area is being developed by several different firms (regional) then what will the total impacts be and how are respective improvements to be apportioned and cost shared?  For traffic flows, what will the impact of the develpment have on the existing infrastructure relative to vehicle counts and intersection movements?  What impact will sanitary flows and water demands have on the existing infrastructure?

KRS Services
www.krs-services.com

While I agree that CVG and KRS are correct - I interpreted the question as to whether or not the off-site drainage would have to be detained.  If I am right - read on. If not - ignore me!

You have to design your system to handle the offsite flows but you do not need to reduce those flows.  

In NJ (USA), the RSIS (residential site improvement standards) call for flows to be reduced to 50% of pre-dev. flows for 2 year storm, 75% for 10 year, and 80% for 100 year.

If you are developing a site with off site drainage contributing, calculate the off-site flow separately and add that number to your allowable flow, i.e., 50% of the 2 year storm (for the area you are developing). Then design your detention system.

Hope that helps, whether I was on target or not with the question!

in many areas, the standard has been, you don't increase the flows leaving your site.  Here in Arizona, 100-year, 2-hour retention is required for all new development in Phoenix.  But offsite still must be "managed".  By this, I mean a plan must be made to convey it through the site without causing flooding.  In fact, the standard here generally is that offsite flows must be kept separate from onsite flows.