Can I cover my entire back garden with an extension of 40sqm?

No. The extension shall not reduce the "Private Open Space" of the back garden to less than 25 sq. m. Hard surface areas to the rear of a house are exempted development, providing they are used for a purpose incidental to the enjoyment of the house and not used for the parking of cars.

N.B. Existing or proposed garden shed/s are not reckoned as private open space