I am sure that if you are only using it for agricultural activity and they are not permanent structures then you can but you may still need to notify your local council. The purpose of greenbelt would allow it in my view under the heading of retaining agricultural use but there are lots of restrictions on buildings and `inappropriate development’: a. to provide opportunities for access to the open countryside for the urban population; b. to provide opportunities for outdoor sport and outdoor recreation near urban areas; c. to retain attractive landscapes, and enhance landscapes, near to where people live; d. to improve damaged and derelict land around towns; e to secure nature conservation interest; and f. to retain land in agricultural, forestry and related uses.