Poly-tunnels: go check !! I believe @Anke you are out of date on this matter.
No I am not. But as you obviously know better, I don't see the point of contributing to the thread anymore. Btw, we have also recently put up a Keder greenhouse, again no planning was required.
The original poster was asking if you could run a garden - I understood probably for food produciton as she asked on a smallholder forum - and as long as you keep the property under separate title deeds there is nothing the planning department can do to stop you running it as a smallholding, in particular growing fruit and veg. Movable poultry structures would also not be a problem, you can set up an orchard etc etc. Unless the whole of Cornwall is now a National Park, you can also put up a Polytunnel, but of course telling "incomers" you can't may just put them off... which is what your previous post was all about.
We have done the above route successfully. People are buying fields everywhere to run as smallholdings. You are just incredibly lucky if you can do so with the field next to your house. Even easier actually if the field is agricultural.