HAVANA — Virtually every square foot of this capital city is owned by the socialist state, which would seem sure to put a damper on the buying and selling of property.

But the people of Havana, it turns out, are as obsessed with real estate as, say, condo-crazy New Yorkers, and have similar dreams of more elbow room, not to mention the desire for hot water, their own toilets and roofs that do not let the rain seep indoors.

And although there is no Century 21 here, there is a bustling underground market in homes and apartments, which has given rise to agents (illegal ones), speculators (they are illegal, too) and scams (which range from praising a dive as a dream house to backing out of a deal at the closing and pocketing the cash).

The whole enterprise is quintessentially Cuban, socialist on its face but really a black market involving equal parts drama and dinero, sometimes as much as $50,000 or more.

These days, insiders say, prices are on the rise as people try to get their hands on historic homes in anticipation of a time when private property may return to Cuba. Exiles in Miami are also getting into the act, Cubans say, sending money to relatives on the island to help them upgrade their homes.

Officially, buying or selling property is forbidden. But the island has a dire housing shortage, despite government-sponsored new construction. And that has led many Cubans to subdivide their often decaying dwellings or to upgrade their surroundings through a decades-old bartering scheme known in Cuban slang as permuta.

Some of those housing transactions are simple swaps. Those the government permits, tracking each one to keep an up-to-date record of the location of every last Cuban. Many moves, however, are illegal and involve trading up or down, with one party compensating, with money, another party giving up better property.

A 1983 film, “Se Permuta,” portrays how complex the system can get: A mother scheming to get her daughter away from a boyfriend she dislikes organizes a multipronged property swap. Of course, the deal, which would have involved about a dozen people and taken mother and daughter from a tiny apartment into a spacious colonial-era house, ends up in a mess, as does the mother’s meddling in her daughter’s love life.

“It’s very Cuban,” Juan Carlos Tabío, who wrote and directed the film, said of his country’s real estate bartering process. “There aren’t enough houses, and families can’t buy them. So they trade.”

Mr. Tabío has no personal experience with changing homes, having lived in the same spacious third-floor apartment in the well-heeled Vedado neighborhood since 1957. Many Cubans live in the same dwellings their families owned before the revolution; others have been assigned units by the state.

But almost every Cuban is either plotting to upgrade residences or knows someone in the midst of the labyrinthine process.

Here is how it works. Imagine a married Cuban couple with two children and a baby on the way who find their two-bedroom apartment in the historic Old Havana neighborhood too cramped. What are they to do?

Well, with the help of an agent known as a runner they might start by locating a bachelor from the countryside looking to come to the capital. They could arrange for the newcomer to move into a tiny apartment in Chinatown and move its residents — who also have a house in Miramar where their elderly grandmother lives — to a first-floor unit they sought in Central Havana. The Central Havana flat is available because the residents have divorced; so the former wife would go to the bachelor’s country house, near where her parents live, while her former husband would go to Old Havana. The Old Havana family that started the whole process would then head to their dream house in spacious and quiet Miramar.

Sound complicated? It is. And the government adds even more hurdles by trying to regulate the swaps with a variety of forms and fees as well as inspections of the properties involved to ensure that they are of roughly equal value.

All trades have to be endorsed by the government, but Cubans say slipping money to bureaucrats increases the chances that deals of unequal properties — as in those that involve money and carry the taint of capitalist yearning — will be approved.

“Under the table, there are all sorts of things going on,” Mr. Tabío said.

The Cuban authorities occasionally make busts, but find the trades difficult to control.

“It’s something people shouldn’t do, but they do and we know it goes on,” said José Luis Toledo Santander, a professor of law and a member of the National Assembly. “It’s like saying you have to stop at the red light and you can’t go until it’s green. You ought to do it, but not everybody does.”

The trading occurs in plain sight. Under the watchful eye of a police officer, hundreds of people gather every Saturday under the ficus trees on El Prado, one of Havana’s grand avenues. Some carry cardboard signs describing their units: the neighborhoods, number of bedrooms and whether there are patios, garages, hot water, private bathrooms and gas supplies. Less desirable dwellings use tanks of gas for cooking and require residents to share toilets with others down the hall.

Ricardo Aguiar, 65, who lives in a two-bedroom apartment in the humble Marianao neighborhood with his wife, daughter, son-in-law and granddaughter, is looking for a more spacious place in Vedado, a popular area closer to the center of Havana. “It’s going to be difficult,” he said, scouring the signs on El Prado recently and checking in with the agents who sit on the stone benches trying to make deals.

“I’ve just started looking, but there are people who look for years and then something goes wrong and they never move,” he said.

Nearby, a woman was working the crowd in search of a first-floor apartment near her current third-floor unit in Central Havana so she would not have to climb so many stairs.

“You have your system and we have ours,” she said, identifying herself only by her first name, Alejandra. “I prefer our system. We don’t have mortgages and so we’re not facing foreclosure like so many of you are.”

Alejandra knows about the foreclosure crisis in the United States because her son lives in Florida and is struggling to make his house payments. “I worry about him,” she said. “If he loses his job, he’ll lose his home.”

Property is sometimes seized in Cuba as well, but by the government, not the bank. Property is taken from those who hop on boats to Florida, although most switch their houses to relatives’ names well before leaving. Those fleeing the island also frequently downgrade their accommodations before going into exile, trading big places for small ones and using the money exchanged on the side to pay for their voyages — the Cuban equivalent of a home equity loan.

Although it is not clear how many thousands of swaps take place annually, some of them involve the same people again and again, as in the case of a woman in her 60s who said she had moved 42 times over the last two decades. “I love to move,” she said. “I can’t live in the same place for a year.”

But her movement is about more than seeking new surroundings. She fixes up each place, then turns it over for a profit, she said in a low voice, declining to be identified out of fear that the authorities might catch up with her.

Moving through the crowd with her is a learning experience. She knows the regulars and can spot the deals. When money is discussed, she and the person she is negotiating with fall into whispers.

“There are so many liars here,” she said, surveying the crowd. “They say they have the best place in Havana, and you get there and you don’t even want to go in. I just stop at the door and say, ‘No, thanks.’ ”

She said she used money sent from relatives who fled to Miami years ago to keep her business going.

“It’s a good time to invest,” she said. “If you have family outside, $20,000 is nothing, and you can get a good place here. If change comes, and we all expect it, then you’re set.”

That is the philosophy of another mogul in the making, who also declined to be identified by name.

Standing in the living room of a two-bedroom apartment in Central Havana that he is renovating, the man estimated its current worth at $20,000, a mint in a country where monthly government salaries can be one one-thousandth of that. If private property ever comes to Cuba, he estimates the price will most likely multiply by five.

Through a complicated transaction, the man recently managed to obtain a historic home in Old Havana that he is also renovating. He said he researched the ownership history of the dwelling because he did not want to find one day that it had been expropriated from an American, possibly leading to a court battle in a post-Castro Cuba. As for his apartment, he rents rooms to tourists, which the government allows.

He is also buying up old chandeliers and other historic furnishings to decorate his units. With most people so desperate for money, he said, he pays next to nothing.

“This is the moment to buy,” he said, referring to Fidel Castro’s illness, talk of change by his brother Raúl and many Cubans’ view that their system, a half century old, will not remain as it is forever.

LONDON

Andrea Chasanow and Nick Gentle picked last summer to search for a house, and that turned out to be a challenging time. Property prices had been booming for years here, but the effects of the global credit crunch were being felt when they saw the house that they wanted and made a bid.

“We were terrified,” said Ms. Chasanow. “I wasn’t sleeping. I wasn’t eating. If you’re a first-time buyer buying at the top of the market, there’s no coming back from that.”

Eventually the couple succeeded in getting the Victorian town house they had sought, in their ideal neighborhood, Primrose Hill. But the search process was rocky, and as it went on, they said, they felt the mood in the market shift. They paid 2.166 million pounds ($4.2 million) for the 2,200-square-foot, five-level house, which needed renovating.

Still settling into their new home, the pair are no longer as anxious about whether their investment will hold its value; they don’t plan to sell for many years. “This is the sort of place we could live in for a long, long time, possibly until we can’t climb the stairs anymore,” said Mr. Gentle, 30, a foreign exchange options trader who works in London.

Ms. Chasanow, also 30, an American who grew up in Chevy Chase, Md., and Mr. Gentle, a Briton reared outside of Birmingham, met in New York in 2003 and moved to Hong Kong soon afterward. They lived there for three years in a rented high-rise apartment that had luxury amenities, including a pool and a concierge, but no charm. “It was like being banished on permanent vacation,” said Ms. Chasanow, a writer who is working on a screenplay. “We could not make it feel like home, no matter what we did.”

Even back then, the couple used to look online at London houses for sale and found one that they loved in Primrose Hill, an upmarket northern neighborhood of mainly terraced houses, about a 10-minute taxi ride from the city center. Although they could not afford to make an offer on that house, they rented near Primrose Hill after moving to London a year and a half ago. “It’s got a little bit of a Bohemian atmosphere, and a really strong neighborhood feel, which we love,” Ms. Chasanow said.

Primrose Hill is cut off from most traffic by Primrose Hill Park, which is adjacent to the village, and Regent’s Park, which is nearby. Its central commercial street, Regents Park Road, is lined with independent boutiques, restaurants and cafes with outdoor seating.

“Primrose Hill retains very much a village-y feel,” said Edward Prickett, an estate agent at the real estate agency John D. Wood, which handled the couple’s purchase. “There are no chain stores here.”

Property in prime areas of London experienced an extraordinary boom in recent years, with values rising 25 percent in 2006 and 26 percent in 2007, according to the real estate agency Knight Frank. As one of North London’s most desirable neighborhoods, Primrose Hill has properties selling for about 1,000 pounds ($1,984) per square foot, said Darryl Jenkins, an office manager at Benham and Reeves, a North London estate agent that operates in the area. Because the neighborhood is small, only 12 to 15 houses come on the market each year, he said.

Last summer, a few months after their marriage, Ms. Chasanow and Mr. Gentle started looking in earnest for one of those houses.

The first place that they fell in love with in early July was a terrace house on a quiet, tree-lined street. Although the house had lost period details, including fireplaces, during a 1970s renovation, it otherwise had “all the requirements,” Ms. Chasanow said, including ample space and a good location.

They were among the first people to view it, but the broker told them that another buyer had already offered 100,000 pounds more than the advertised price of 2 million pounds ($4 million). Ms. Chasanow and Mr. Gentle bid 2.15 million pounds ($4.3 million) and were rejected, and then increased their offer to 2.175 million pounds ($4.35 million) and were accepted.

Thrilled with the deal, they found a surveyor and a lawyer, but about two weeks later, the agent called to say they had been “gazumped,” a common term in British real estate that means someone had outbid them. They no longer had the house.

“We were just devastated,” Ms. Chasanow said.

After a vacation, the couple started looking again in August. By now, worries about the subprime crisis in the United States had started to affect confidence in the market, they said. Agents whom they had not been able to get on the phone a few months earlier were suddenly barraging them with text and e-mail messages.

Among the new listings was a terrace house in Primrose Hill advertised for 2.1 million pounds ($4.2 million). On a dead-end road off of Regents Park Road, the house was about the same size as the one they had lost, but its fireplaces and original moldings were intact. Initially they bid 2.15 million pounds ($4.3 million) and were rejected. Then, in a sealed-bid auction, they offered 2.177 million pounds ($4.34 million) and were surprised to find that they had won.

“We never thought that we’d get it,” Mr. Gentle said. “Because of the experience trying to get the other house, we felt we didn’t have a chance.”

Their elation was dampened, however, when they went to get their mortgage, which had been agreed to in principle a few months earlier. Because of the downturn in the market, their mortgage broker had raised the interest rate by nearly half a percentage point.

“That was upsetting,” Mr. Gentle said. “We’d maxed out in the sense that we’d done our calculations and knew what we wanted to spend and we did not want to go over it.”

The couple decided to take advantage of the same flexibility in the system that had cost them their first house. With their offer accepted, they asked the owner for a discount, or, in local lingo, they “gazundered.”

“It’s the flip side of gazumping,” Ms. Chasanow said. “It’s not considered very nice. It’s just the way things are done in England.”

In the end, the owner agreed to take 11,000 pounds ($22,000) off the price, a fraction of the final cost but enough to ease the couple’s fears.

“It made us less angry about the mortgage,” Ms. Chasanow said.

After closing on the house in November, the couple moved in December. They made a few changes — painting a few rooms and damp-proofing the basement — but have substantial work left to do. Old carpet still covers the stairs and ground floor. The patio garden is overgrown, and the basement needs paint and some repairs.

To make better use of the space, Ms. Chasanow and Mr. Gentle plan to change the house’s layout. For instance, the want to remake the basement — now a separate one-bedroom apartment — into a kitchen and dining room. They also want to expand the small attic bedroom, and replace the steep windy staircase leading to it with a wider one. Redoing the bathrooms is a priority; they have two and a half bathrooms, including the one in the basement, but none have showers.

“It’s a little weird to spend all this money and not be able to take a shower,” said Ms. Chasanow, who acknowledged that she felt some sticker shock at the scale of London house prices. But finding a place to renovate was always part of their plan. “We really wanted a house that we could put our mark on,” she added. “We didn’t want to pay for anyone else’s taste.”

Although the credit crunch — and resulting higher interest rate — has cost them in monthly mortgage payments, Ms. Chasanow and Mr. Gentle said that their timing was fortunate. If the market had not cooled, they believe they would have been outbid again. “I think because of the credit crunch, people hung back and said, ‘We’ll see where it will go,’” said Mr. Gentle.

Moreover, they now are pleased that they lost the bidding war for the first house, since they like the house they bought much better. “We’ve thought about sending the people who ‘gazumped’ us flowers,” Ms. Chasanow said.