The question I get asked most, usually by someone standing in front of a mirror looking faintly betrayed by their own reflection, is what smart casual actually means. My honest answer: one properly made piece doing the heavy lifting, paired with something relaxed, so you look like you tried without looking like you're off to a tribunal. A good blazer and decent trousers with trainers instead of brogues. A sharp shirt with jeans that have a proper fit rather than a sag. That's the whole trick.
Where it goes wrong is mixing too many registers at once. Smart casual is not the outfit equivalent of hedging your bets, so don't throw a structured blazer over joggers with a drawstring you can see from across the room, and don't wear your actual work trousers, the ones with the crease ironed in by a machine somewhere in a factory, with a hoodie, because the two halves will just argue with each other all day. Trainers that are clearly gym trainers, scuffed and grey at the toe, will undercut even a lovely jacket in about four seconds. The whole point is that nothing should look like it wandered in from a different outfit entirely. Before I learned that, I turned up to a Thursday lunch in a blazer, flat trainers and leggings that had gone slightly see through at the knee from wear, and a friend told me, kindly, that I looked like I was between two meetings that didn't exist.
Before you buy anything in this kind of outfit, check the fabric weight on the blazer or shirt, because cheap smart casual separates are often cut from something thin enough to see your bra strap through under decent light, and that's not a look, that's a mishap. Check the trouser rise too, standing and sitting, because a low rise that's fine standing at the till will gape the second you sit down for coffee. Feel the lining if there is one. A blazer with no lining at all will crease into a map of somewhere unpleasant within an hour of sitting down.
This is the outfit for the whole stretch between nine and whenever you finish, weekday lunches, a casual Friday that still has a client call in it, early autumn through to spring when you need a layer but not a coat indoors. The good ones earn themselves by day three of wear, when the shirt still sits flat at the collar and the jacket shoulders haven't gone soft, and you realise you haven't adjusted anything since you left the house.