Yes, it's a percentage of your existing property or a maximum based on cubic feet.
Check with the local planners - You will not be able to sell your house if it turns out it was required.
With reagrds permitted devevlopment rights as stated above, the LA will also take into account any previous development on the property (This may not be relevant) however, if you have extended previously, it's likely you will have used your permitted development right.
Also - Is the propoerty listed, in a conservation area etc etc.
Permitted development counts for nothing if you are.
Check before doing anything.
LA'a are generally sympatheti