Permitted development- loft conversion

[TropicalPotatoes]

Can anyone help with this.

We want to convert our loft. Can anyone tell me if the added 50sq meters, under permitted development, is measured externally or internally? I've been given different answers every time I speak to my local planners and it's driving me crazy

I believe it's more commonly externally. We're going through the same thing and this appears to be the general consensus, although it could vary from council to council.

Externally.