If I recall they have approval to go to 35m pax but not the facilities in place to go beyond 25m, I guess this is them putting in that missing capacity (which they will need before too long).
If so then there is an interesting contrast in approach here - Stansted will spend £600m to get roughly 10m additional capacity while Luton is spending £100m to get about the same (excluding the more recently announcd £200m people mover).