Owning your own home

I'm putting some decking in my garden. It's about a 5x5m garden with some lawn in the middle. However, the lawn is unusable - either a muddy bog in winter or a concrete type wasteland in the summer so we thought we'd try to do something nice looking this summer.

Most of the lawn will be replaced with decking. I've got some questions about this later, but for now I was hoping that someone could help me with working out if I need planning permission.

Planning guidelines state you need permission if:

The decking is no more than 30cm above the ground
Together with other extensions, outbuildings etc, the decking or platforms cover no more than 50 per cent of the garden area.
None of the decking or platform is on land forward of a wall forming the principal elevation

I'm fine for those except the 50% one. It will cover >50% of the back garden, but likely not 50% of the total (front and back) garden area. Does anyone know how to interpret this?