Taken with existing extensions, the proposed rear extension to allow for the construction of a first floor would result in a significant increase the habitable floor space of the property. The existing extensions have already substantially increased the size of the original, modest, vernacular dwelling and the proposed extension would result in a combined total of new development which would considerably exceed the specified limit of 30% of new habitable floorspace, as stated in Policy CO17 of the NYM Local Plan. The cumulative increase in the overall habitable floorspace would have a detrimental impact on the mix of dwelling types needed to sustain balanced communities within the National Park; therefore the proposal is considered to be contrary to the Authority's adopted planning policies.