The deepest national box
There are no drawings available here, but they aren't needed. Just assume the top of the box is exactly the same as the national super or brood box, but 165mm deeper than the super and 89mm deeper than a standard brood box, making it 304mm deep. It's as simple as that. There may be small discrepancies in the figures which are caused by the B.S. specifications being in imperial sizes.
The 14 x 12 has been in existence a long time, although it has only recently come into common use. It is known as the B.S deep box, where many beekeepers refer to the standard brood box as the "deep" when it isn't.
Having used many kinds of hives and frames in excess of 50 years, I have to say I dislike the 14 x 12 for a number of reasons. The "thinking" is that the "modern" bee needs more room, but I struggle to understand what the so called "modern" bee actually is. I can only assume it is thought to be more prolific than bees previously were. This doesn't make too much sense to me, as I haven't detected any change in the prolificacy of bees in my time. Quite frankly I think it is little more than a fad that I suspect will disappear. It could be argued that 14 x 12 are too large for non - prolific bees.
I'm happy to accept that other beekeepers can use whatever they wish, but my objections to 14 x 12 are:-
For those who haven't got 14 x 12, but are told how wonderful they are, I suggest you back off a bit. Handle them at other people's apiaries on several occasions before being persuaded to change to them. If you are satisfied they are an improvement on what you already have, then use them. If your bees are prolific, then consider changing to less prolific bees. You may find they are easier to manage, give the same or more honey and need less feeding.
You must do what you think is best for you. Don't let me put you off.