Terminal upgrades are far, far easier to plan and implement than a new runway.
Not sure I agree with you entirely on that one Shed
The only interface with the existing operation when R2 was constructed was the crossing points on R1. Otherwise it was just one big builders compound where the contractor could phase things as they wanted to for the most efficient construction schedule.
However, the most difficult phase of the runway development was the work required to get to the stage where the planning permission was granted. Work started in 1989, an application was made in 1993, the Inquiry was in 1995, permission was granted in 1997 and operations commenced in 2001.
Getting planning permission for a terminal should be a lot easier - just look at LHR. The planning Inquiry for R2 at MAN was around 100 working days; I seem to remember the MAN T2 Inquiry was a couple of days.
If you are building a new terminal on a new site such as T2 at MAN, again it will be mostly a contained building site during construction.
AFAIK the new T2 at LHR was also effectively one big building site which was isolated from the rest of the operation for most of the construction period as all the T2 traffic had been decanted elsewhere
But as soon as you start working on a terminal upgrade, you are working in an operational area and therefore you have to maintain access to enough areas to ensure that passenger operations can continue. In order to do this the plan has to be split into many phases which takes longer and costs more. The MAN TP is like this because unlike LHR there is much less scope for decanting and this will be a major challenge to ensure that the demand can be handled all the time. We know the rough sequence of events but as Shed posted there is a lot of detail yet to come out and as the design progresses we may see some changes.
This addresses Viscount's point about the different timescales for the MAN TP and LHR T2 and has already been pointed out by Shed.