40 sqm exempt planning

Hi,

If a house had a 20 sq metre extension built in the 1980s (with no planning) and this is now knocked to build a 39 sq metre extension, does the 39 sq metre extension still qualify for the 40 sq metre planning exemption? Or is the knocked 20 added to the new 39 to give 59 sq metres, meaning that planning permission is required?

There are no complications like neighbours as it is to the rear of the property and out in the country with no neighbouring houses in sight.

Thanks,

If you knock the original extension and construct 39 in total you will still be exempt. Aslong as you adhere to the other requiremets such as height, remaining open space and location of windows relative to boundaries, and location of the extension (rear only).