After reading many of the responses to this thread I see allot of "what ifs" and "that wouldn't be allowed in my area" responses, we must keep in mind we all live in different ares and these areas may or may not have code amendments that trumps what the NEC may or may not allow, this is up to the person doing the installation to check with there local AHJ and state laws to know what options they might have, including what their utility allows.
So when we answer a question we should base our answer on the NEC not what is allowed or not in our area, we can offer an opinion of it, but it does the OP'er little good if it doesn't apply to his/her location.
Like I said, The NEC allows the installation in the OP if other problems that has to be addressed in existing installs, such as:
Size of the existing service and the new load calculation, which will be a problem either way you do this install, off the meter or adding it to the existing panel
Double wires on the meter lug, could be solved by a tap box below the meter base, or changing the meter to one that allows double lugs on the load side, using a double lug to single pin if you can still find them .
For new installations it will always be a lower cost using the 230.40 Exception #3 method as it allows for less disconnects, wire and other materials, as also the load of the out building is not added to the dwelling service panel so even breaker spaces are freed up,
But in existing installations one must weigh the options to see which one is more cost effective and requires the least repairs to make when doing it, sometimes a home owner might not want drywall removed if it is possible to do so in another way even if it does cost a little more.
This is why the NEC has these little exceptions because it allows us to have options so we can chose which is best for our customer.
Also when we address our local codes we must know the NEC and how it is supposed to apply, one of the more common mistakes I see with the above use of 230.40 Ex. 3 is inspectors tend to try to use requirements in article 255 to disallow it, remember if you are ahead of the service disconnect you are not in article 255, you are in article 230, also even if they get the correct article they tend to not read what the code says, like the example of Grouping of disconnects that only apply to each structure, or that a disconnect is not required if the conductors do not "enter" a structure, they will call these conductors feeders, feeders are after the main service disconnect not before, again remember we are in 230 not 255, a meter can is not a service disconnect, it is nothing but a wide point in the service entrance conductors, SEC's enter it and SEC's leave it.
On another note is the grounding electrode requirements, I have had inspectors try to say I have to bond between two buildings to connect the two grounding electrode systems together, this is not what the code says, again each building or separate structure is required to have all the grounding electrodes bonded together, there is no requirement that requires us to bond these two Grounding Electrode Systems together from separate structures, each building or structure is its own stand alone system.
So be sure if you have an AHJ trying to disallow this 230.40 exception 3 allowance, that they are calling the right codes and properly reading them if they are.