I've asked the DCLG and got the following response, which I translated to mean that changes of use are not major development unless on a site of over 1ha:
"The statistician has asked me to refer you to the following guidance (the most relevant part shaded in red), from page 13 of the PS1/2 guidance notes at https://www.gov.uk/government/publications/district-planning-matters-return-ps1-and-ps2:
“Major developments - definition
For district matter applications, ‘major development’ means development involving any one or more of the following;
- The provision of dwelling houses where –
o The number of dwelling houses to be provided is 10 or more: or
o The development is to be carried out on a site having an area of 0.5 hectares or more and it is not known whether the number of dwelling houses to be provided is 10 or more.
- The provision of a building or buildings where the floor space to be created by the development is 1000 square metres or more: or
- Development carried out on a site having an area of one hectare or more;
Where a site above one hectare is subject to a change of use application, it should be coded under major development and not as a change of use (Question 20). Any other applications for change of use should be captured under Question 20 and not under the relevant major/minor development category.”
