The world of limousines and aftermarket extended-wheelbase builds is certainly an interesting one, with panel fitment that's suspect at best, and the unforeseen mess that may lie underneath.  So it makes sense that one particular affluent european individual decided that rather than look to an agency to fulfill their extended dreams, they'd go to the manufacturer themselves.
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The result of this inquiry is what Audi is referring to as the A8 L extended, a 20.9ft long ultra-luxury vehicle with seating for six and all the fit, finish and structural integrity you'd expect from a factory-built vehicle.  Now featuring six doors and an additional row of seating, the car features a larger glass roof, revised styling from the A-pillar back, and electronically adjustable rear seats.  To maintain braking performance while carrying the additional weight, 15" brakes from Audi's S8 have been fitted.
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Audi says that they have already received additional requests for custom product similar to this A8 L, meaning that if you've got the cash, Audi just might make the one-off hyper luxury car of your dreams.

Read the full press release below.
  • Premium manufacturer develops and builds 6.36-meter (20.9 ft) sedan to special customer order
  • One-off specimen meets same standards as production model
  • Comfortable and spacious seating for six 
Audi puts the customer first, however out of the ordinary their requirements may be. An example of this is the Audi A8*: at the request of a European customer the premium brand has built a first one-off version of the sedan in king-size format. Measuring 6.36 meters (20.9 ft) in length and with a wheelbase of 4.22 meters (13.8 ft), the A8 L extended* affords top-class comfort for passengers occupying all of its six seats. A concept that generates customer: Audi has further requests for the custom product already.
Experts invested almost a year in developing and building the exceptionally long luxury sedan. The result was an exclusive one-off specimen that meets the high standards of the production model in every domain. The major challenges concerned the safety and rigidity of the extra-large body, as well as the styling of the silhouette.

The Audi A8 L extended benefits from the low weight of the production model’s body, which uses an Audi Space Frame design and is made almost entirely from aluminum. For the built-to-order version the engineers extended the side sills and center tunnel using extruded aluminum sections. Inside the tunnel, these also assist with the air routing. A double rear bulkhead separates the passenger compartment from the luggage compartment; the roof incorporates additional aluminum profiled tubes and cross-braces. These measures provide torsional and flexural rigidity almost on a par with the A8 L*.

The representative XXL sedan in discreet moonlight blue, metallic offers 1.09 meters (3.6 ft) more wheelbase and overall length than the Audi A8. To maintain a harmonious roof line, the entire body from the A-pillar back has been reworked.

The experts embedded a 2.40-meter (7.9 ft) glass panel in the roof skin to enhance the sense of spaciousness. The sun blinds – one for each row of seats – prevent overheating of the interior in strong sunlight. The headlights and rear lights incorporate small side marker lights, as is prescribed by law for vehicles of this length.

All passengers face the direction of travel in the Audi A8 L extended. The six seats are upholstered in extra-supple Valcona leather, in the color velvet beige. Rows two and three feature electrically adjustable individual seats. The third row has a continuous center console, a Rear Seat Entertainment display and a cool box. 

The one-off A8 version is powered by a 3.0 TFSI engine developing 228 kW (310 hp), delivering up to 440 Nm (324.5 lb‑ft) of torque. This propels the sedan from 0 to 100 km/h (62.1 mph) in 7.1 seconds and up to an electronically governed top speed of 250 km/h (155.3 mph). An eight-speed tiptronic transfers engine power to the quattro permanent all-wheel drive system. The 19‑inch wheels in 15‑spoke design conceal the brake system of the Audi S8*, which assures outstanding deceleration of the over six-meter-long (19.7 ft) luxury sedan, with its unladen weight of 2,418 kilograms (5,330.8 lb).