You may need planning permission for a kitchen extension. It normally depends on the size and position.
Several option are possible.
As the name suggests this simply utilises the side space at the back of your house and doesn't extend any further into the garden than your existing building. The side roof usually has to slope down(can be glazed) to the height of your neighbours wall(usually 2.1 metre).
Often don't need planning permission for this but do ask your local council
Here you are extending beyond the original rear walls into the garden.This creates tons of extra space. May not need palnning permission depending on size.
This is the most expensive option but creates masses of new space. It combines the side return and rear extension to substantally increases your new living area.
Usually need planning permission.