Yes, whenever it is NOT required by some local reviewing agency. Otherwise, local detention is almost never a cost effective alternative to regional detention, IMHO.
I've ran into situations like this before and as RWF7437 said only when it is not required by the local reviewing agency. However, it all depends on where your project is located in the watershed as well. For example if your project is located near an outlet to a lake or river or....attenuating your runoff may "match peaks" and possibly make the downstream conditions worse.
Read regs/drainage code carefully, it is either a change in impervious surface area or change in runoff rate that triggers detention requirements, typically. If your project is below those thresholds, then detention would not be required.
As RWF and Ryb said, it would depend on requirements set forth by the regulatory body (i.e. city, county, etc.)
Examples where you may not have to detain include: if you were tying into a downstream storm drain system and you could prove that it had the capacity to handle your increased runoff, you may not have to detain. Or if you were outfalling into a channel and could prove that you weren't adversely impacting downstream property owners (or get consent from d/s owners), it may be allowed (doubtful, though).
The real issue is that a church in my local community wants to construct a 50 ft x 75 ft youth activity building and the city wants an engineer to analyze the runoff. The church's property is 13 acres more or less. The difference in the C values are pre = 0.419 and post = 0.422. Would you say that the change in peak runoff is insignificant if the time of concentration is the same for the pre and post?
If you're using a single Tc and composite C value for the entire 13 acres, this will tend to mask the effects of the 50x75 roof runoff, which will have a much smaller Tc and therefore higher peak flow.
PSMART makes a very good point. If using a single Tc still causes an increase in site runoff (which sounds like it would be negligible), you may be able to do a simple analysis of the downstream receiving system (whether this is an open channel, storm drain, barrow ditch etc.) and show that it can handle the increase from the additional impervious area.
In some jurisdictions if you disperse roof runoff onto pervious ground surface or with drywells it can be considered disconnected impervious area and would be modeled as pervious, and your calculated site runoff may show no difference in pre- and post-developed for the 13-acre property.
As bltseattle says, the requirement for local detention is almost always governed by some local code. For your case, where you are only converting 0.09 acres of 13 acres ( this is less than 0.6%) from pervious to impervious area the effect is probably trivial and is certainly less than the error inherent in the Rational Method.
Even if some storage is required by your local authority it could not be very much and should be easily and cheaply provided for somewhere within the 13 acres you have available.
