How do you extend joists?
Using more joists bolted on to the existing (but now shorter) joists.
That's not a very good solution, and I would be surprised if your BCO's favour this over other methods - which they can't really do, as they are not able to dictate how something is achieved, but rather just check that the end result is satisfactory.
They seem pretty happy with it to be honest. A lot of people seem to be opting for the rsj to be hidden in the ceiling, so it's a case of fit the joists on one side into the web and then extend the joists on the other side into the web on the other side.....
It's either that or joist hangers.