Planning permission is rarely required for decks, providing the following criteria are not exceeded. However, it may be prudent to obtain a Certificate of Permitted Development if there is any doubt. If asked, your local authority will almost certainly suggest that you submit a planning application. They make a charge for this!

Garden or Timber Patio Decks.
No permission is normally required (Check with your local authority Planning Department) if >>>>>

The requirements for planning permission for decking and deck projects are briefly set out below.

Decking Projects Requiring Planning Permission include the following (There can be other reasons)

If the deck is situated within 20 metres of a highway. (This means the closest part of the footpath - not the road!)
Where the deck is at 1st floor level of the house or above.
If any part of the deck construction exceeds 3m in height from original ground level to the top of the balustrade.
If the structure would affect the amenity value or privacy of neighbouring properties. (For example if your deck is situated where it intrudes on your neighbours privacy and normal use of house or garden) This is becoming increasingly important!
If you live in a flat or maisonette - Totally different rules to living in a house as far as garden structures are concerned!
If the deck is attached to a listed building or situated in a conservation area or National Park.

If your deck is at ground level, then it will not require planning permission, for you are simply adding a hard surface in your garden. You are entitled to do this anywhere in your rear garden - or all over it if you wish!