NOVATO -- Coming off a recent $139 million initial public offering of stock, Ultragenyx Pharmaceutical, Inc., (Nasdaq: RARE) is doubling the size of its headquarters as it enters latter stages of research and development of some of its treatments for rare diseases.

As of mid-May, Ultragenyx (415-884-2800, is set to occupy all the 43,500-square-foot office building at 60 Leveroni Ct. The company signed a five-year lease of the 24,846-square-foot first floor of the building on Feb. 28. At the end of last year, the company had 59 employees, including four to five who work in leased laboratory space at The Buck Institute for Research on Aging, also in Novato, and the rest in 20,500 square feet of second-floor offices.

"We’re fully maxed out here with offices and space," said Tom Kassberg, chief business officer of Ultragenyx.

The new space at this point is expected to accommodate 50 to 60 more employees and suffice the rapidly growing company's needs for 12 months, he said. After that, Ultragenyx may be shopping in Novato or elsewhere in Marin County for bigger quarters.

Yet for the foreseeable future, the company isn't planning to set up its own production facilities, in Marin or elsewhere. Ultragenyx currently uses multiple third-party manufacturers to make research and testing quantities of its products.

"We feel that, at this stage, it is cost-effective approach, rather than to invest in facilities," Mr. Kassberg said.

Another Novato-based developer of rare-disorder treatments, BioMarin Pharmaceutical, has spent hundreds of dollars per square foot to establish and expand its Novato production facilities. Because the BioMarin and Ultragenyx use similar biological technologies during production, the costs for a local plant likely would be similar, Mr. Kassberg said.

Ultragenyx has disclosed five clinical research programs in various stages. The furthest along is rhGUS, or UX003, an enzyme-replacement treatment for Mucopolysaccharidosis (MPS) 7, or Sly syndrome.

With no approved treatments currently, MPS 7 affects about 200 people in developed nations and can cause enlarged liver or spleen as well as joint stiffness. Ultragenyx's candidate treatment started phase 1/2 trials on up to five patients in December. Last week, the company reported that an emergency treatment on a 12-year-old led to reduced size of the liver and spleen in 14 weeks.

Founded in 2010, the company has raised $244 million so far, including the IPO, a $30 million series A round in June 2011 and a $75 million series B round in December 2012.

The company's stock price has risen from $21 a share at the IPO to $49.79 Thursday.

Mark Carrington and Haden Ongaro of Cornish & Carey Commercial Newmark Knight Frank represented Ultragenyx in latest lease at 60 Leveroni. Steven Leonard represented property owner Condiotti Enterprises.