dufferman
Journeyman Pro
Roof height is probably your biggest issue. Think if the building is less than 2 metres from a boundary the maximum roof height is 2.5 meters. More than 2 meters away from a boundary you can have a pitched roof up to 4 metres. Size wise you can't go above 50% of the land that came with the house, you also have to include other outbuildings in that 50% as well. I have not looked for a while so rules on permitted development may have changed so worth checking online.
Yeah, good point re the height:
https://www.planningportal.co.uk/info/200130/common_projects/43/outbuildings