Back Forty in the City

But others have a positively suburban idea of outdoor space. For them, it’s not worth having unless it’s large enough to plant a garden, to have a barbecue for a dozen of their best friends, or to host a croquet match, or maybe even all of the above.

A glance skyward at the city’s many apartment towers reveals no shortage of apartments with small balconies suitable for a chair or two and al fresco bicycle storage. Real outdoor space, though, is difficult to come by.

A recent search for perhaps the most unusual kind of outdoor space — a terrace or garden larger than the apartment proper — produced only about two dozen examples. They ranged from a $12.5 million penthouse with 3,000 square feet of terraces to a $289,000 studio with a 400-square-foot garden.

Brokers say that outsize terraces call out to a special breed of buyer. Outdoor space can drive up the apartment’s price and make it harder to sell in a down market, but it adds a distinctiveness that can help the property maintain its worth. “Outdoor space increases the value of an apartment,” said Jonathan J. Miller, the president of the appraisal firm Miller Samuel. “It’s an amenity like a fireplace or an extra bathroom that you add to the basic apartment. And it can redefine it.”

The percentage of apartments with any kind of outdoor space has increased in recent years, mainly because developers have built many more apartments with balconies and terraces. While only 19 percent of the apartments that sold in 2000 had outdoor space, the number grew to 23 percent in 2010, Mr. Miller said.

First, some definitions.

Balconies are typically 6-by-10-foot boxes that hang off the side of a building. Terraces sit on top of a roof or on an extension of a building. Space on the ground floor tends to be an earth-floored yard or a patio of some kind.

Appraisers say that not all outdoor space is created equal. “The value added depends on the appeal of the outdoor space and on the rarity of it,” said Michael Vargas, the president of Vanderbilt Appraisal.

Balconies on the Upper East Side, for example, tend not to affect the selling price, he said, because so many of the postwar buildings in the neighborhood have them. Outdoor space in a downtown loft building, on the other hand, can command a substantial premium because it is so unusual.

Mr. Miller says outdoor space is typically valued at 25 to 50 percent of the interior space. So, for an apartment worth $1,000 per square foot, outdoor space would range from $250 to $500 per square foot.

A second-floor terrace that overlooks a loud and busy street would be closer to the 25 percent mark, while space that is on a higher floor and offers more privacy would edge up to 50 percent. “If it’s so loud that you can’t talk and there’s an inch of soot on all your furniture,” Mr. Miller said, “then it’s not really functional, because you can’t really enjoy it.”

He added that in general, if the outdoor space is larger than half the size of the apartment, “there’s a diminishing return on the price per square foot. It becomes excess space.” That’s because there are limits to how much buyers will spend on, say, a one-bedroom or a two-bedroom, regardless of how much outdoor space it might have.

In many prewar buildings, the top floor was not always prized space, instead relegated to the superintendent’s apartment or servants’ quarters. That is the case at 780 West End Avenue, where Michael Franco, a vice president of the Corcoran Group, has a listing for a $975,000 penthouse apartment of about 750 square feet with a 950-square-foot wraparound terrace.