With good-quality treated timber, proper ventilation underneath and a bit of maintenance, a well-built deck should give you many years of use. The things that shorten a deck's life are poor drainage, no airflow under the boards, and never re-treating it. Build quality matters more than the timber grade — most decks that fail early do so at the frame, not the surface.
Realistically: a proper clean once a year, and re-oiling or re-treating every year or two depending on how exposed it is. Left alone, timber greys off, and in a damp shaded spot it will get slippery. If that sounds like more upkeep than you want, composite decking is worth considering instead.
Usually not, if it's low-level. Raised platforms, balconies and verandas can need permission, and decking that lifts the platform significantly can also raise overlooking and privacy issues with neighbours. If your garden slopes and the deck needs to sit high at one end, that's worth checking before we start. We'll flag it if we think it applies to your project.
