The appetite for online fashion and luxury shows no signs of flagging, and Net-a-Porter.com’s biggest challenge this year is keeping up with that demand. Natalie Massenet, the site’s founder and chairman, will move the company headquarters from London’s Bayswater to larger premises — 40,000 square feet — at the Westfield mall in northwest London. The pioneering site, which was the first to sell luxury fashion around the globe, services its clients much like a high-end specialty store, and has aggressively leveraged the Internet with projects such as an Apple iPhone app and virtual trunk shows. Massenet also plans to invest in the expansion of its distribution centers in the U.K. and the U.S. In mid-February, the company will relaunch its online, full-service Wedding Boutique, and later this year will unveil a second iPhone app, for theOutnet.com, its separate, discount sales site.