How is my planned 2-storey rear extension limited by the 50% limit of the total land area?

This 50% limit of the total land area is only applicable if your project falls within the usual permitted development (PD) rights. As you are building a 2-storey extension, PD rules will not apply and your project will be determined under a standard householder planning application.

If you were to build a single-storey rear extension under PD rules, the 50% limit would operate in accordance with the wording:

“Development is not permitted …… if …… the total area of ground covered by buildings within the curtilage of the dwellinghouse (other than the original dwellinghouse) would exceed 50% of the total area of the curtilage (excluding the ground area of the original dwellinghouse”