"BBB" debt has a historic default rate of 0.35%, Fitch reports.

A portfolio that boasted an occupancy rate of nearly 94% was one factor in Fitch Ratings' declaring Boston Properties' outlook "stable." The rating service notes the company's three largest tenants, which account for 12% of its total leased space, are the US government, Citigroup and Ernst and Young. While the Big Four accounting firm carries no rating, Fitch rates the federal government a "AAA" tenant while Citigroup is rated "AA+." The average lease term also is seven years, Fitch Ratings reports, which is among the longest in the industry.

On the other hand, Fitch Ratings notes the portfolio concentrated primarily in four markets, with 60% in New York City and Boston. San Francisco and Washington, DC account for another 31% of the company's square footage, Fitch Ratings reports, making it vulnerable to regional economic downturns.

While the rating service has placed a "negative outlook" on the office sector, it also is concerned that 10% of the REIT's annualized base rent is generated by one property--399 Park Ave. in New York City. In addition, Boston Properties' development pipeline of $839 million is nearly of 9% of its total market capitalization, Fitch Ratings adds.