I don't know about recently, but I think in 2011 or so there was a change which means you don't need planning permission for a Loft Conversion as long as it fits within certain criteria. You still need building regs, party wall agreement etc.

at least that's my understanding based on us thinking of doing one ourselves.

I think one of the reasons though is prices are so expensive now if you need another room or so, it is cheaper \ feasible to do the loft, where it will be too expensive or out of reach to move up the property ladder as such.