The 'quota', I think refers to this, I reckon - if you want to build a single storey extension in the back garden, you can do so without planning persmission, so long as it is does not extend the overall floorspace of the house by more than 10%. Of course this is a one-off allowance - you can't keep extending by less than 10% every year! I think there is a cut off date - if the house was extended before the 1930s(?) that extension won't use the quota.