We want to buy a beautifully renovated, below-grade co-op in Carnegie Hill, and we're planning to get a mortgage with a 30% down payment (at least until we sell our single-family house out of state). The first lender we approached denied our mortgage because a doctor's office shares the lobby of the building; they were willing to lend to us in that building, just not for the first-floor apartment. We're now starting over with a different lender. Why is this so difficult? How do we get around this?

You’re running up against a part of New York City real estate that’s often surprising for out-of-towners, and that’s a set of Fannie Mae guidelines that applies to condo and co-op buildings, as well as individual borrowers, our experts say. Since many banks sell mortgages to Fannie Mae, they follow the government-sponsored agency’s criteria for what constitutes a “safe” loan to make. And thus a perfectly qualified New York buyer may have trouble getting a mortgage for an apartment in a building that, for example, doesn’t have enough money saved up in its reserve fund.

In your situation, the fact that the co-op is below ground is probably not the main stumbling block, unless that lender has some specific internal rule, experts say. Likewise, a doctor’s office in the building --- even on the same floor as the co-op and the lobby -- likely isn’t the issue. “If the doctor’s office is part of the co-op, the lender is making an error as the only rule regarding commercial space is the no-more-than-20%-commercial-space rule,” says Rolan Shnayder of H.O.M.E. Mortgage Bankers, referring to a Fannie Mae rule for apartments in buildings where more than 20% of the space is non-residential.

However, if the doctor’s office is not part of the co-operative but is sharing the common space, that could be a problem for Fannie Mae, Shnayder says. Similarly, there may be a snag if the place you want to buy was once a professional space and then was converted to an apartment, notes Robbie Gendels, a senior loan officer in the New York City office of National Cooperative Bank, since it could technically be counted as commercial space -- and not something that would qualify for a home loan.

Regardless, your decision to seek out another lender is the right move. Smaller, private lenders will be more likely to make an exception for a qualified buyer than a big, brand-name bank. Indeed, portfolio lenders -- which keep mortgages on their books, rather than selling them to Fannie Mae, and thus are not beholden to the agency’s rules -- may lend to you. Note that if you’re borrowing $625,500 or more -- that is, a jumbo loan that doesn’t conform to Fannie Mae’s guidelines in the first place -- you’ll have to go to a portfolio lender, notes Peter Lucia, the Brooklyn branch manager for Mortgage Master. “Every portfolio lender is going to have a different take on this,” he says.

It may be difficult, but if the co-op is your dream home, it pays to shop around with other lenders. “If there are sufficient windows and you love the apartment,” Lucia says, “go for it.”

Leigh Kamping-Carder is a senior editor at BrickUnderground.com, the online survival guide to finding a NYC apartment and living happily ever after. To see more expert answers or to ask a real estate question, click here.