I would like to convert our attic into a bedroom. Where do I start? The roof structure needs changing, do I need planning permission?

First you will need to get a professional construction person in to inspect your attic house and see if the attic and the support structure can hold additional weights,

But you will need a building permit from your city and or county planning, and of course if you live in an area with historic or building restrictions you will need to be sure you meet those restructions also.

You will have to have a complete layout of the plan to file with the permit and of course they will have to inspect each section of the work, electric, framing ( plumbing if any plumbing changes) and roofing.

So to answer your question, yes you will need formal bulding permits