For the purposes of the Greek authorities and particularly the taxman, thousands of extra rooms converted from basements and balconies, not to mention entire houses, simply don't exist.

But many of these owners nationwide are now opting to declare them under a new amnesty law intended to enforce some regularity in the housing system -- and to raise some much-needed cash for the Greek state.

Ministers believe they can raise one or two billion euros by offering a 30-year amnesty to owners of illegal buildings in return for a fine, although this is a drop in the ocean of the country's crippling 350-billion-euro debt.

Greece has about one million illegal buildings, out of a total of seven million homes, according to a study by the Athens Polytechnic.

The new amnesty law, which was passed by the Greek parliament in September, gives owners until the end of November to declare any buildings dating from 1955 which have been built or extended without planning permission.

"I think many people will come forward," said Paris, one of a number of civil engineers in Athens who have found new business in helping owners update their building plans so they bear some resemblance with reality.

"Because the law also introduces an important proviso -- that no house can be sold or passed on without a certificate from a civil engineer confirming the accuracy of the plans."

Owners have another incentive to come clean: if they don't declare their illegal buildings, they risk paying a fine every year that is even more than what they must pay now, or see their homes demolished.

Under the amnesty, a 100-square-metre illegal home would invoke a fine of between 9,000 and 36,720 euros according to the area, the age of the building and other particulars of the case, paid over several months.

But Kostas Dialos, the president of an association representing property owners, says the plan is blackmail.

"It has only been drawn up to raise money. Citizens shouldn't have to pay for the negligence of the state, which has failed to come up with any proper policy on urban planning," he said.

Dialos claims that most of those who have built their homes illegally "are people of modest means who, to find somewhere cheap to live, have no option but to buy land where construction is prohibited".

Elias Beriatos, a professor of geography and land management at the university of Thessaly, agrees this is an important factor.

"The Greek state has failed to do its job properly when it comes to housing. For example, there has never been a programme of social housing," he said.

As a result, the explosion in the urban population between the 1920s and 1970s led to a construction free-for-all in the towns.

A more recent wave during Greece's economic boom has also seen illegal holiday homes spring up all across the coastline and in wooded areas.

The government says most of the money raised from the fines will be set aside into a "green fund" tasked with demolishing illegal buildings in protected areas, which will not be subject to the amnesty.

However, environmental campaign groups Greenpeace and the WWF have criticised the plan, saying similar initiatives in the 1980s failed to make an impact.

Beriatos said the illegal rural developments have benefited from inadequate land registers and zoning regulations, while the labyrinth of red-tape required to obtain a legitimate construction permit is a disincentive to be honest.

"A system of patronage" does the rest, he said.

He also warned that the amnesty plan, conceived under pressure from Greece's foreign creditors to raise funds, offers no long-term solution.

"This new law regularising properties, agreed with such haste, only continues a history of short-term policies which have no consideration for the long-term management of the land," he said.