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If proposed works fall within the thresholds of “permitted development rights” then a formal planning application is not required.
The thresholds for extensions for “permitted development” rights are contain on the following link for you to check:
You will see that one of the thresholds for rear single storey rear extensions must not extend beyond the rear wall of the original house by more than eight metres if a detached house; or more than six metres for any other house. Extensions of more than one storey must not extend beyond the rear wall of the original house* by more than three metres.
So provided that the extension is 3 meters or less then you would not have to apply for planning permission by reason of the size of the extension alone, but you would have to check that you comply with the rest of the thresholds on the above link.
Building regulations approval and a completion certificate will be required for the extension:
Most loft conversions fall within permitted development rights but you will see that the “permitted development rights” for loft conversions are contained on the following link for you to check:
Building regulations approval and ideally a completion certificate will be required for the loft conversion:
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