Layout change flat - planning permission

I'm looking to swap my kitchen and bedroom around in a 3 bed maisonette,
moving my kitchen/living to the large bedroom creating a larger living space and smaller bedroom.

I understand i'll have to apply for building regulation approval for the kitchen work but what is the rule regarding flats, do I need planning permission for this?

I'll not be altering the external appearance in any way although the flat is located in a conservation area, and i'm not sure if this changes things.

Any ideas would be much appreciated.