Polytunnels are a great addition to an allotment. They offer a protected environment to grow on seedlings in the spring, provide ideal conditions for heat loving vegetables like tomatoes, chillies, aubergine, and sweet peppers in the summer, and extend the harvest season to as late as November. It is even possible to use a polytunnel to grow cold tolerant vegetables like lettuce and perpetual spinach to harvest salad leaves throughout the winter.
There is a wide variety of polytunnels to choose from, including options like:
One door, or two doors
Polytunnels that have roll up sides
Polytunnels that bury the plastic in the ground (trench versions), or nail the plastic to wooden battens along their base (base rail versions)
Of these options, I have chosen a base rail version with non-roll up sides, and a single door (with a false door at the other end). This is a relatively simple specification, and I have not felt the need to upgrade as these options suit what I like to grow. The only thing I would change is to make the polytunnel larger. It is the most production area of my allotment, and the most enjoyable too.
The reason I chose a base rail version is that I did not want to risk damaging the buried plastic along the sides of the polytunnel when weeding. Roll up sides would be an advantage in the summer to reduce heat, but I like to grow heat loving vegetables inside the polytunnel, and therefore this is not an issue for me. On warm days I simply keep the door open, and at the other end, the false door has a mesh panel that provides good air circulation.