BMW is hard at work developing its next-generation 3-Series, prototypes of which have been spotted testing around the globe for the past couple of years now.

Already we’ve seen prototypes for the regular 2013 BMW 3-Series sedan, the first of the new line, as well as its Touring wagon sibling plus a new Gran Turismo model and 3-Series ActiveHybrid.

Now we have spy shots of a 3-Series Long Wheelbase model, which has been spotted testing in Germany though it's being designed exclusively for the Chinese market.

You’ll recall that China already has a unique BMW 5-Series Long Wheelbase model but soon buyers looking for something a little bit more economical will have an option. The 3-Series Long Wheelbase sedan is expected to be unveiled at the 2012 Beijing Auto Show next April before going on sale in the Chinese market shortly after.

According to onlookers, the prototype seen here is around 4.0 inches longer than previous prototypes for the regular 3-Series sedan.

Designing a long wheelbase model is not simply a process of adding a couple of extra inches to the wheelbase. Individual design features along the roofline and the side view of the car are required to ensure that the long wheelbase model still offers the right balance of proportions.

Otherwise, the cars will be much the same as the rest of the 2013 BMW 3-Series fleet. For more information including engine details and release dates for the other models, click here.