it's measured as a whole, but excluding the house and curtilage.
Now, the interesting bit is that it also includes rented land, which is even non-adjacent... I think this point has to be argued with the planners a little. But PD is exactly that... permitted. The planners have to find a pretty good reason not to grant, and it's a national permission, not a local one.
so, if you only have 11 acres without house and garden then, in theory and evidenced practice, you could rent a block of 5 acres nearby and be ok. The limit for building size for PD for agri use is HUGE! So don't worry about that. 465 square metres I think... and bonkers tall. We built the biggest thing we could afford, which as 30ft x 60ft x 12ft tall at the eaves (about 16ft at the apex). It is waaaaay under the PD maximum. What's even more daft is that 2 years after each PD application, you can apply again, for another building, as long as the total area of both buildings is underthe 465 sq m limit. In reality, this means that at the end of next year we could build another identical 30ft x 60ft shed and still be under the limit. After that, you're into full planning permission.
you may still be granted permission for a shed under the 12.5 acrea, but it'd be full planning.