Karlcavanagh

Hi all, I am looking to build an extension to the side return at the rear of my terraced property. I have had conflicting information re which route I should take, l obviously don’t want the expense of full planning permission when 42 day prior approval will be adequate. Please advise 6mx2.4m extension to rear side return Flat roof 3m tall at the eaves. Warm roof construction with large skylight. Overall height 3.2m including parapet. 300mm insulated cavity wall replacing the existing dividing wall between my neighbour. Party wall agreement in place. I believe 42 day prior approval is adequate am I missing something. Happy to provide any info I might have missed Thank you in advance Karl