Market trends, also a factor in a home’s value, can be hard to pin down, especially if prices in a specific neighborhood rise quickly. Because it can take two or more months from the time a contract is signed on a home to the time appraisers learn of the offer, such fluctuations are not always reflected in the available comps. Appraisers can make what is called a time-adjustment to account for market shifts, but it may be hard to get a lender to accept it, Mr. Heskel said.

The best thing a homeowner or broker can do to help the appraisal process is to prepare a one-page sheet for the appraiser that outlines the changes and repairs that the home has undergone since it was bought, Mr. Miller said.

Given the complexities of New York City real estate, having appraisers familiar with the neighborhoods in which they work is crucial. Unfortunately borrowers don’t always get them. When Shane Koss, a 38-year-old musician, and his wife, Dagmar Kostkova, a 38-year-old farmers’ market coordinator, had their offer accepted on a one-bedroom in Greenpoint, Brooklyn, in February 2012, they didn’t anticipate any problems. Then Mr. Koss received a call from the appraiser. He needed directions to Greenpoint.