Many single-storey rear extensions can be built without a full planning application, as long as they stay within limits on depth, height and the share of the garden they cover (sample note; rules differ from place to place).
The limits change on corner plots, in conservation areas and for houses that have been extended before. A neighbour consultation may still be needed, and building regulations always apply.
At a first site visit we look up the planning history of your address and tell you which route applies, before anyone draws anything.