Timber decking differs from a conventional patio, most significantly due to elevation and general height of construction. For all but the most simple, low level deck, it would be worth checking with your local planning department that permission isn’t required.
As well as contacting your local planning authority, it would be worth having a chat with your neighbours about your plans. Neighbour objections are the most common reason for planning being refused or for enforcement notices after completion. Local authorities can insist that structures are dismantled and removed where consent should have been obtained.
This is a guide to when planning would be required (but is not a definitive list):
Where the deck is situated within 20 metres of a highway
Where the deck platform is more than 300mm (1ft) from the ground
If the structure would affect the amenity value or privacy of neighbouring properties
If the deck is attached to a listed building or situated in a conservation area or National Park
Building regulations should be assumed to apply to every structure that requires planning permission.
In addition to the situations set out above, other restrictions have been known to apply, including limitations to the overall deck area in relation to the existing property or garden area and the constraints of established building lines. For example, in England (after 1 October 2008), surfaces, including decking, are not permitted to cover more than 50% of a property's garden.