Kia, one of the fastest-growing brands in the U.S., plans to invest $100 million in its 18-month-old West Point, GA, manufacturing complex to make way for local production of the new Optima sedan.

Adding Optima to West Point, which Kia first announced in April, will raise the site’s annual production capacity to 360,000 vehicles annually by 2012.

The facility currently builds the Sorento and Hyundai Santa Fe cross/utility vehicles.

Like its sister company Hyundai, Kia has experienced sharp growth in recent years. According to Ward’s data, Kia’s U.S. market share grew from 1.8% in 2006 to 3.1% in 2010.

So far this year, the auto maker has increased its U.S. share to 3.8%, with the Sorento leading the way.

The new-for-’11 Optima also has been a key piece of its success through the first five months of this year, racking up 29,518 deliveries on slim inventories.

In May, the redesigned car accounted for 7,431 sales, or 5,000 more than its predecessor.

To accommodate the extra production, hiring of an additional 1,000 employees begins later this month and will bring the facility’s employment to 3,000 people.

Details of the expansion include a second, 5,400-ton (4,899 t) transfer press for the body shop; additional robots for the welding shop; expansion of the paint shop; extension of the production line and additional equipment in the general assembly area; expansion of the railroad spur; and more employee parking.

Kia opened the $1 billion West Point site, the auto maker’s first North American production facility, in November 2009.