This lays down the planning law. Note that planning law is founded on the idea of just a few 'use' classes. You are allowed to _use_ a property (be it house, field etc - the 'property is the square footage on the map) in the ways laid down in the Town and Country Planning act. A change of use is described as a 'development' in law. This idea can be seen in the first emboldened line below. No change of use = no development = no problem in law.
Town and Country Planning Act 1990
(2)The following operations or
uses of land shall not be taken for the purposes of this Act to involve development of the land—
(a)the carrying out for the maintenance, improvement or other alteration of any building of works which—
(i)affect only the interior of the building, or
(ii)do not materially affect the external appearance of the building,
and are not works for making good war damage or works begun after 5th December 1968 for the alteration of a building by providing additional space in it underground;
(b)the carrying out on land within the boundaries of a road by a
F2. . . highway authority of any works required for the maintenance or improvement of the road [
F3but, in the case of any such works which are not exclusively for the maintenance of the road, not including any works which may have significant adverse effects on the environment];
(c)the carrying out by a local authority or statutory undertakers of any works for the purpose of inspecting, repairing or renewing any sewers, mains, pipes, cables or other apparatus, including the breaking open of any street or other land for that purpose;
(d)the use of any buildings or other land within the curtilage of a dwellinghouse for any purpose incidental to the enjoyment of the dwellinghouse as such;
(e)the
use of any land for the purposes of agriculture or forestry (including afforestation) and the use for any of those purposes of any building occupied together with land so used;
https://www.legislation.gov.uk/ukpga/1990/8/section/55