In 40 seconds
A new garden deck in the UK typically costs roughly £60–£150 per square metre installed, so a mid-size deck of around 20m² commonly lands between about £1,200 and £3,000+ depending on material, sub-frame and access. Softwood timber is the lowest-priced option from around £60/m² installed but lasts roughly 10–15 years with regular upkeep; composite costs more up front at around £100–£150/m² installed but lasts 25–30 years with very little maintenance, which is why over the long run it often works out lower in cost than re-treating timber. For most gardens, decking no more than 300mm above ground needs no planning permission, but go higher and it becomes a raised platform that may need an application, with a balustrade required above about 600mm. The honest answer is always a range, because it depends on your deck size, material and ground.
Most decking guidance is published by companies selling boards or fitting decks, so the numbers tend to be optimistic and the rules glossed over. The pages below give honest cost ranges, compare composite and timber fairly, explain how long each lasts, and set out the planning and building rules — before you take a single quote.