Contrary to first impressions, this isn't a facelifted Audi, but an all-new one. Other than the gearbox, there's hardly any part bigger than a nut or bolt that hasn't be entirely renewed.
Let your eyes run across it for longer and the newness becomes apparent. This is the third generation since the 1996 original. In that time the A3 has progressed from a rounded, chubby puppy to something a whole lot sharper and more thrusting.
That's nothing compared with what's gone on underneath. Every engine is pretty well entirely new. So's the suspension, the whole body structure, the electronics, the seats, and all the supporting systems like the air-con. It's very, very rare that any manufacturer turns out something that carries over so little from any existing car.
It's so new because it's the first car to use the VW Group's new modular transverse-engine components set (or MQB). This astonishingly versatile kit of parts will be available with a huge range of engines, suspensions, wheelbases, tracks and so on. It'll make everything from the next-gen superminis to the Passat, Audi TT and various crossovers.
This isn't old-school ‘platform sharing' because such wide variations are possible between the different models. That's the cleverness of the MQB, and the reason it had to use no existing parts.
In all, VW, Audi, Seat and Skoda will eventually be churning out about 10 million cars a year using the MQB jigsaw. Think about that. Ten million. Every year. That's a car every three seconds.
But what really matters to you and me isn't this industrial angle for its own sake. It's how it affects the cars themselves. Firstly, developing each individual model costs less. And second, buying in such unimaginable bulk drives the price down.
For Audi, that money is spent elsewhere in the car. For instance, the new A3 has aluminium front wings and bonnet, and a cast aluminium front subframe. Even the centre armrest bracket is cast magnesium. These expensive lightweight parts are expensive but help make the car better to drive.
And they've thrown money at the interior too. The new A3 has absolutely gorgeous quality in its materials and touch-points. It feels solid in here. Heavy, even.
But because it's been engineered from the ground up to use lightweight parts - especially in the nose of the car - it feels the very opposite of heavy when you drive. It moves with a nimbleness the old A3 never had. You can punch the front end into a corner and it hardly understeers.
In, through and out of a bend, the handling is light-footed and playful in a way that no mainstream Audi has never managed. There's even some feedback from the steering. The damping could impose a bit more discipline on long-amplitude body movements, but we were on the softest of the three available suspension options. There's also a magnetic ride option, which is probably the answer. I haven't tested it on this car but it does a great job on other Audis.
And the ride's decently supple too. There's hardly a trace of the stiff-legged wooden jiggle that you suffered in the old A3.
The top engine for the moment is a sweet-natured 1.8 TFSi petrol with 180bhp. The turbo makes it punchy and keen all the way through the rev range. They claim it'll get to 62mph in 7.2 seconds.
But the one most people will buy is the 2.0 TDI with 150bhp. Again it's an all-new engine, and the quietness is pretty astonishing. It chortles away gently at urban speeds, hums nicely as you accelerate round the dial to 5000rpm, and disappears almost entirely at a cruise. Nothing wrong with its delivery or freedom to rev, either.
For people paying car tax - that's most A3 buyers - the critical figure for this engine is 106g/km.
Less weight doesn't only help the handling, then, but the performance and economy too. A new A3 is at least 100kg lighter than an equivalent BMW 1-series, although to be fair an Alfa Giulietta runs it close.
A3s always had the best interior quality in the class, and they always had efficient engines. But as a car to drive, this is the first time ever that an A3 has ever really been competitive with the best rivals.