One of the usual rules about the car business is that the path to creating an exciting car (which is also legal, safe, marketable and manufacturable) is far too steep for any but the professionals to climb.

Britain's Autocar magazine is challenging this idea, by teaming up with TWR Design to conceive, engineer and build, as a full-size model, a new kind of compact family car. It will be unveiled in November at the prestigious Autocar Awards ceremony in London. At that stage its external shape, interior package, cabin styling, chassis and suspension designs, engine and transmission details, materials - even its production schedule - will have been laid out as if the model were being prepared for a 2004 model-year launch.

The project is real in every sense, except that there is no blank space in a car maker's production schedule waiting for it. But even that could change.

The design team thrashed out ideas in an early meeting at TWR's new design and prototype works at Worthing on the south coast - and alighted with surprising ease on an idea for a practical but stylish lightweight second car, of which millions are sold every year to families across Europe as superminis.

But this Autocar-TWR vehicle would be no catch-all workhorse. It would be a more stylish, more novel machine than a VW Polo or Ford Fiesta - economical and nippy enough to be practical, eye-catching and adventurous enough to be the kind of thing an enthusiast would want to own, especially at a proposed showroom price starting from £10,000 to £12,500, depending on model.

Radical decision: the team decided to make the vehicle a pure three-seater, slightly shorter than a Ford Ka or Mercedes-Benz A-class. Why only three seats? Because they believe such a car could be amazingly roomy for its small exterior, and uniquely useful to the ubiquitous European family of four.

Look at the practicalities: this family's breadwinner drives a conventional saloon or MPV in which he totes the family after work, at weekends or on holidays when they want to travel en masse. When this car is away at work, the family needs no more than three seats, but a good deal more agility and economy: hence a lightweight three-seater. Given the way teenagers grow, it would be handy if each seat could comfortably accommodate a six-footer. These can.

Early calculations also show that the car can have a greater and more versatile load-carrying capability than comparable conventional saloons, and can carry two mountain bikes inside when toting only two occupants. Thanks to its unique seating layout, it affords the third person better visibility and more sociable seating because he looks between the heads of the two front occupants and sits less than 18in behind them. The close-coupled seating would work uniquely well for a rear-mounted baby seat, too.

The idea for TWR Design to build a full-size prototype car, with Autocar as the client, was born in the office of Craig Wilson, managing director of TWR Group. The automotive design and engineering concern has been built up over 25 years by the chairman, Tom Walkinshaw, from motor racing beginnings. With typical clear-mindedness, Wilson picked up Autocar's perennial desire to understand the process by which cars were conceived, and decided to do something about it.

Nowadays most of TWR Group's £400 million-a-year business is as a consultant, designing and producing information on the production of cars for motor manufacturers around the world. TWR also keeps a strong foothold in racing through the Arrows F1 team. Last year the company acquired the former Daewoo design and prototyping centre at Worthing, and set a course for even greater involvement in the motor industry.

It is now one of a very few consultancies worldwide which create production-ready cars - developed from a client's tentative ideas - and also engineer the means of making them. At the first Worthing meeting, which consisted mostly of animated talk, TWR's team compiled what they termed a list of product attributes, just as they would for a new client. The car was to be:

- A three-seater, for '95 percentile' occupants (meaning that only five percent of adults will either be too big or too small to fit comfortably)
- Simple, lightweight and high-tech
- Easy to manufacture, perhaps from computer-designed kits
- Small, but extremely practical
- Reconfigurable and upgradable
- A new type of family car
- Sporty and fashionable - a motoring enthusiast's car.

At this stage, the team had only the vaguest idea of the position of the engine, although a transverse front-drive location seemed limiting, given that the three-seat layout looked like promising more versatility. One of TWR's people hit upon the idea of a '3x3' marketing message: three people, three-cylinder engine.

It was decided to proceed to the next stage with that, without laying it down as a hard point. Other proposals included joystick steering (too radical for a 2004 model-year car), a keyless ignition/entry system (not popular with the Autocar people, who have tried them all), head-up display instrumentation (it works brilliantly in the Chevy Corvette) and a stop-start engine mode to save fuel in traffic (agreed by all).

After a tour of the enormous and remarkably well equipped Worthing facility, the Autocar group departed for home, to await the call when TWR's designers had developed their proposals. Two weeks later, the team gathered at TWR's Leafield HQ, just north of Burford in Oxfordshire. In the virtual reality studio, there were no fewer than eight concepts to evaluate, divided broadly into layouts with two front seats and one in the rear, or McLaren F1-style, with a centrally seated driver and two passengers to the side and slightly behind.

Here, the Autocar group confronted the first difficulty of the consultancy client: having to decline well produced and credible design work. An Autocar committee that included Editor Rob Aherne, Editor-in-chief Steve Cropley and senior staff writer Colin Goodwin decided that two front passengers and one behind made a more sociable, accessible layout. That ruled out half of the concepts. It seemed a combination of crassness and waste, but if you're building just one car you have no alternative. Designers have to be both highly creative and thick-skinned, it seems.

They'd work the chosen two proposals up for the next meeting, to see which would finally get the nod. Four days later the final concept , featuring external luggage panniers, was chosen and will now be developed for the November deadline. You'll be able to see the results soon...

Related sites: TWR