i would ask if there are any restrictions regarding size , materials etc . if youve NOT increased the size it could/should be classed as a refurbishment not a new structure .i wouldnt worry to much , even if you are required to get planning permission and the others are not ,wave the human right flag . you can always apply for retrospective planning if needed , get your club,fed ,union and fellow allotment members on your side . stay posotive. and good luck.