Planning
That picture from the masterplan does not quite tell the whole story though. It's worth remembering that this was published in 2007 by MAG and there's no indication it was updated. It is therefore 17 years old now.
If we cast our minds back to when the terminal was upgraded, the original plan was for the departures to be built as now, with the original arrival portacabin structures remaining as domestic arrivals with a much larger two storey international arrivals hall being built in a similar position to now. But then the economy crashed in 2008 and MAG then decided to change the plans. The new arrival hall was eventually built substantially smaller and the existing structures were removed. This resulted from a second round of planning applications in 2010. MAG believed the previous design was not sustainable in the new environment and could cram the new combined arrival facilities into a much smaller flat roofed footprint. This is what was eventually built with the old arrivals building being cleared out in the process.
They clearly can't knock the new arrivals hall down and build the original design now. That wouldn't be valid given the subsequent planning approvals anyway. There is nothing in the planning approvals that allowed building new structures in between the two buildings at this stage. Clearly the unstated intention was to leave that space free for easy future development.
Despite the southern sector being green belt and the attempted judicial review of the original terminal planning approval I think future development in this void space has a reasonable chance of going through. But it surely will have to go through the lengthy planning process I'm afraid. It will also be at risk of political point scoring delaying and adding extra cost to the process. I think from reading the tea leaves that they are planning to manage next summer with the facilities as-is, plus the current £5m round of improvements of course.
It's worth remembering that many things from the master plan and original planning applications haven't come to pass, such as the section 106 agreement for the airport to fund an hourly bus service for a minimum of 12 hours a day (presumably negotiated away during the collapse in air traffic) and the southern car park, which was to be bis served and located roughly underneath taxiway bravo with taxiway alpha becoming a branch off bravo. That never came to pass and would have been expensive to operate. With the cargo hard standing now built, it's hard to see that coming to pass in that location.
All of this information can be found on the BCP planning website in the Christchurch section. Put "airport" in the address box and scroll back through to 2007 and 2010.