When Tamar Zacheim and her husband heard from their agent about a two-bedroom condo for sale in Georgetown, they immediately sat down and crunched the numbers.

But unlike other young couples, the Zacheims weren't calculating mortgage payments and taxes. Instead, they first wanted to determine how far the property was from an Orthodox Jewish synagogue.

They punched in addresses on the popular Google maps Web site and the numbers that came up were fantastic. The condo is about one-third of a mile from the Orthodox Kesher Israel synagogue. The walk would take only about seven minutes.

"We have to be in a one-mile radius and obviously the closer the better," said Zacheim, the new mother of a baby daughter.

Like other Orthodox Jews who strictly follow the tenets of their faith, the couple must walk to their synagogue on the Sabbath and holy days. Such observant Jews are not allowed to drive, take a taxi or ride a bike during the Sabbath. In some instances, they cannot even carry a handbag or push a baby stroller.

For that reason, homes immediately surrounding the region's few Orthodox synagogues are hotly pursued by members of the community, a fact not always well known to home sellers, agents and other prospective buyers who are interested in the same properties.

"In a way it's a shame, because most people can live anywhere and yet they might be bidding on a home that's highly, highly desirable to an observant family," said Minka Goldstein, an agent with Long & Foster Real Estate Inc. Of course, she said, the most competitive bid is going to win a house, "It's a business transaction. But it can be very hard for the family."

The area's super-heated real estate market has made finding a home challenging for almost any buyer. But it has become exceedingly difficult for Orthodox Jews who are relocating to the area, or local families who are trying to trade up to larger homes.

"They're really tied to very specific areas," Goldstein said. "They can't say, 'Oh, we'll just move further out and find something we can afford.' "

The problem is exacerbated because most of the region's Orthodox synagogues were established decades ago in neighborhoods that have become expensive. Kesher Israel is in the heart of Georgetown. Several other Orthodox synagogues are tucked into Silver Spring and Potomac. In all, there are fewer than a dozen Orthodox communities in the metropolitan area. And, for the most part, the real estate market in those areas has become tight and expensive, with the few mid-priced homes that go on the market routinely receiving multiple offers.

Elanit Rothschild is the housing coordinator for Kesher Israel, which provides real estate listings and housing tips on its community Web site. She said the search is much easier for singles and young couples, especially if they want to rent rather than buy.

When she moved to the District two years ago, the synagogue gave her a list of rentals in the area, and she signed a lease in a building that houses other members of her community.

But she said when Kesher Israel's young people are ready to marry and start families, they usually have to move on to another synagogue or settle for smaller homes.

"They weigh the pros and cons and if they decide remaining at Kesher is more important, then they give up the space," she said.

Settling for smaller homes can be complicated, said Goldstein, who is Orthodox.

She said that observant Jews need large kitchens to accommodate two complete sets of dishes required by religious dietary laws. Some even create second kitchens in the basement to be used during Passover week. Families tend to be large, she said, and they value a big, open dining room where relatives and guests can gather for Sabbath meals.

"The dining room is key," Goldstein said. "Observant families have other special considerations besides location."

Families who can't find suitable homes near an Orthodox synagogue face a long walk to services. But even more problematic, they risk the possibility they've gone beyond the eruv, the symbolic enclosure that surrounds Orthodox communities.

An eruv consists of an unbroken border of walls and gates. In most areas, telephone wires stretched between poles are the de facto "gates" that define boundaries, but buildings, fences and sea walls can also serve as borders. Where there's a break, the community strings up cord or wire to complete the enclosure.

Once inside, members are able to carry objects, such as a diaper bag, when they walk on the Sabbath and other holy days. They are also permitted to push a stroller or a wheelchair.

Outside the eruv, Orthodox Jews aren't supposed to carry anything -- so they must walk to Sabbath services without so much as a house key.

Unknown to most, the region's eruvim (plural for eruv) have constantly been changing and expanding in the past few years to encompass larger and larger areas, as housing prices drive members farther from their synagogues.

Kesher Israel's rabbi said he is negotiating with Virginia officials to extend the synagogue's eruv across the Potomac River and into Rosslyn, which is a long but manageable walk.

"More and more people are being forced to move further out," said Rabbi Barry Freundel, a national expert on eruvim. "You build them (eruvim) and you have to keep expanding them. The eruv has been in a constant state of change, just constant."

The housing crunch in Georgetown, Potomac and Silver Spring -- sites of most of the region's synagogues -- has revitalized the few Orthodox synagogues that are located in less-pricey areas.

Shari Diamond and her husband are members of Kesher Israel in Georgetown, but now that they have a toddler and a new baby they're looking for a house in the District's Shepherd Park, near Ohev Sholom Talmud Torah, an Orthodox synagogue along 16th Street NW.

"It's really picking up, with a lot of young families like us locating there," she said. "It's still possible to find something and be part of an established community. We also wanted to stay in the District, and this is our only other option."

Other families have given up trying to find affordable housing near the established synagogues and have banded together to form new centers of worship. Under Jewish law, 10 men together form a minyan that can conduct communal religious services.

Freundel said he has helped several families who have relocated to neighborhoods in Northern Virginia, which has many fewer established options, to find ways to gather and pray together on the Sabbath.

But these satellite pockets of worship can seem like a last resort to many Orthodox Jews, who say they love the sense of community they get from living near synagogues.

"When you're walking you see your neighbors and it's really pleasant," said David Shaool of Potomac, who is hunting for a new home on Long Island, N.Y. Non-observant "Realtors would show me a house that was a mile and a half away from the synagogue and say, 'Oh, you can walk that.'

"But sometimes I have to walk three times in a 24-hour period for services. Do the math. That's like walking nine miles and once or twice with a stroller," he said. "It's supposed to be a happy, uplifting day -- not an endurance test."

Shaool's 4,900-square-foot house in Potomac is on the market as the family prepares to relocate. Not only is it within walking distance to three Orthodox synagogues, it's also filled with details that observant Jews appreciate.

For instance, his refrigerator and stove are designed with special switches that turn off the interior lights for the Sabbath, so that householders don't inadvertently flip on a light when they open the door, an action proscribed under Jewish law.

Given the house's location and features, it would be an ideal home for an Orthodox family. But legally, sellers cannot discriminate against buyers based on their religion, so Shaool can only hope that another Orthodox Jewish family puts in a competitive bid.

"There's sometimes grumbling in synagogues that sellers raise the prices knowing that buyers are desperate for that location," agent Goldstein said. "But really it's just following the market as a whole. Prices are going up everywhere."

Sometimes, though, everything falls into place. The Zacheims made a bid on the two-bedroom Georgetown condo and got word the next day that their offer had been accepted. They not only are within easy walking distance to Kesher Israel, but also have family nearby in Silver Spring.

And best of all, Zacheim said, there are other young Orthodox families in the same condominium, so they will have company on their walks to Sabbath services.