A spacious new premium hatchback joins the BMW 3 Series range in the shape of the Gran Turismo, Matt Kimberley, PA motoring writer, drives it.

Following in the footsteps of its big brother the BMW 5 Series Gran Turismo, the 3 Series GT has added a third body style option alongside a saloon and Touring estate.

The new GT is 200mm longer even than the 3 Series Touring, despite looking more like a sort of inflated hatchback. It's only 75mm shorter than a 5 Series. Most of that length is in the wheelbase with a 110mm increase, but there's no hiding the extra 90mm slotted into the rear overhang. That, and a 81mm height increase, gives the GT a visual balance weighted heavily towards the rear – a matter of taste but usually a trait of prestige cars.

If you're wondering why it's so big your question is answered as soon as you slide into one of the rear seats. There's loads of legroom – 75mm more than the Touring and more like what you find in a 5 Series. You can't tuck your toes under the back of the front seats owing to the electric gubbins within them that occupies that space instead, but there's still more than enough space.

Extra space behind the B-pillar is what the GT is all about. Along with the extra passenger space it brings 25 additional litres to the boot next to the Touring, which rises to 100 litres more when the two cars are compared with their rear 40/20/40 split seats folded down. Again it's closer to what you find in a 5 Series.

So the obvious question is why you wouldn't just buy a 5 Series. At £1,300 more expensive than equivalent 3 Series Touring models, the GT is reasonably pricey and it's possible to buy a 5 Series for roughly the same outlay, but the Gran Turismo models seat you higher and capture the faintest hint of the essence of a sport utility vehicle, which some people will make a beeline for.

There is nothing radical about the engine line-up, which is familiar to anyone reasonably well acquainted with BMW. There are two diesels to begin with – a 141bhp 318d with low CO2 emissions of 119g/km and a 181bhp 320d – and three petrols in the shape of a 181bhp 320i, 241bhp 328i and 302bhp 335i, the only six-cylinder engine available.

The latter isn't available in entry-level SE, only in Modern, Sport, Luxury or M Sport. Modern is the sharp and stylish choice, Luxury is plusher, Sport slightly more aggressive-looking choice and M Sport the harder, harsher one that British buyers love for its looks but not its ride quality. Among other things it gets more supportive seats but they're also firmer, bordering on uncomfortable especially in the back where such hard seats serve little real purpose. It's better to stick to Modern or Luxury where the equipment better suits the GT's remit.

Still thinking of the car's natural GT character the optional eight-speed automatic gearbox is a winner for long-leggedness and relaxed cruising. Its shifts are very fast but smooth and forgiving, meaning that whatever engine is under the bonnet the car pushes forward with a relentless quality that's amplified by the more powerful engines. In truth even the more powerful diesel, the 320d, struggles to move the automatic 3 Series GT along with real pace, but the 335i does a convincing job with an enjoyable engine growl. Turbocharging softens the note a little relative to that of the old normally-aspirated 302bhp straight-six found in the pre-2007 3 Series, but it still throbs nicely.

The faster you go the better the basic ride quality becomes, because while at slower speeds sharp bumps send a slightly harsh impact through the cabin, at speed things are much more composed. There's an initial bit of body roll in any of the user-selectable driving modes before the GT settles into a stable cornering attitude.

So why would you buy it? If a regular 3 Series doesn't quite cut it, a 5 Series is too ordinary and an X3 lacks the right executive image, the 3 Series GT offers a blend of all three to provide a top-drawer solution as a mid-size executive shuttle. Rear passengers will thank you for it, too.