Plot holders are responsible for the structures on their plots. The site rules say they should be structurally sound and new sheds should be approved by the committee. We have some brick sheds in a very sorry state, which plot holders may take on at their own expense - changing locks, fixing doors/roofs etc.
We are self managed, and only charge what we pass on to the council in rent, which is under £1 a perch, so it doesn't seem unreasonable for people to look after their own sheds.