Good question. Hadn't really thought about that before. I'd probably start with the full 16psf and see how crazy the design gets. If it's not too bad, then stick with it. If you can't make it work (or it only work if you put in a steel brace or something equally ludicrous for a 150sf addition) then back off a little.

I'd probably look at your various load cases and apply the 16psf in each case in a manner proportional to the windward and leeward pressures you calculated. For example:

Case 1:

Windward Pressure = 9psf

Leeward Pressure = -3psf

Total Effective Wind Pressure: 12psf

Windward is 9/12=0.75, so apply a windward pressure of 16*0.75=12psf

Of course this gets more complicated with taller buildings or buildings with multiple diaphragm heights where you need to divide it up a bit more based on actual elevation pressures. But this should start you in a reasonable direction.

Thanks for correcting me, MotorCity. I was thinking there was a statement about a combination of windward and leeward in the code. That's the implication (it only matters if windward+leeward is less than an effective 16psf), but it's not written that way.

I disagree on the shielding comment in this case. Shielding is meant to apply to other buildings and terrain features. There is no 'other building' in this case. It's a 150sf addition. That tells me it's probably a utility room or a small sun room. Maybe a bedroom? It's part of the other structure. If it were separated by a short breezeway or set 5' away, then sure - no shielding. But if it's built by cutting a door opening in the wall of the existing building and building three walls to enclose the space, it's an addition and part of the the existing regardless of how you want to resolve the loads. Checking to make sure a 3-sided diaphragm works in that situation is a matter of expediency that is usually acceptable in small residential or other RC II structures at strength level design (which is where the 16psf comes into play). Have to be a little more careful with serviceability since you can could get some unintended load transfer.

If they went through the trouble of detailing the connections, the OP would be justified in analyzing the entire structure - existing and the addition - as a whole. That effectively means that the the existing structure would 'shield' the addition in one direction.

Of course we haven't seen a plan and we haven't been read in on the construction type, so maybe I'm not being conservative enough in light of the lack of information.

The minimum 16 psf wind load is just that....a minimum, whether its windward, leeward, or a combination of both, a minimum load of 16 psf needs to be applied to the structure. It sounds like this is a horizontal addition to the building and you are looking to use a smaller wind load due to it being shielded by the existing building. Unfortunately, you are not permitted to consider shielding.