One could approximate sheet flow with a single or group of cross sections then change the single or group of cross sections to reflect the additional foot print. If you cannot approximate then you would be looking at a 2-d analysis which seems way too much for this scenario; but, I am not familiar with your local criteria or regulations.

JSiteDznr,

Curious:
Does this not qualify for an Standard Plan? (Residential construction less than 1.0 acre disturbed) or an exemption? (less than 5,000 feet of disturbance)

Otherwise:
If there is no channel on site - one would probably need to establish a downstream analysis point where there is channelized flow (swale, ditch, closed drainage system). This should be a point of agreement with the reviewer at a pre-application meeting.

This is one of those cases where you need a parameter in the existing condition to compare with Post condition. In this case, you have post Water surface elevations (WSE) but you don't have pre WSE's to compare. You would need to identify the downstream point of analysis, preferably where there is concentrated flow leaving your site. This point of analysis should be same for both pre and post. Then assume a "pseudo" channel/swale at the analysis point for the pre and perform your flow calculations. The existing "pseudo" channel or swale may not have much of a defined channel geometry, but you need to approximate.
Explain this approach in the report and let the reviewer comment otherwise.

Can you pop a raingarden in the front or backyard and call it a day, or use some amended soils and take credit for the higher void ratio thereby meeting the pre to post rate requirements.

The other way is to determine the Pre and Post runoff volume. Then determine the difference. If the difference in runoff volume is less than 2%, you can include in the report that no significant downstream impact is anticipated. If the reviewer wants you to still provide SWM system, just use Rain Barrel at the downspout to capture as much roof runoff to offset the increase.

Also look at your point of interest in your analysis. Where is this site located within the watershed? If you look at the larger basin, you may be able to demonstrate that the increase in runoff will be negligible compared to the overall basin. Does this development drain to an existing stormwater management facility downstream?