converting detached garage into habitable room

[jimhet]

I'm looking to convert my detached garage into a habitable room/office and have been on the uk.gov planningportal website which seems to suggest that since October 2008 garage conversions generally fall into permitted development. I'm wary of contacting my LAPD for so called advice [we have history] so would be grateful if anyone is able to expand on the planning portal litrature.