The adjustments will be the first since 2011 and come as prices of real estate have slumped during the crisis. A condition for adjusting the property values is a change in market prices of more than 10%.

The announcement comes after the recent debate on the proposed real estate tax saw complaints that the current values in GURS's database do not reflect reality on the ground due to the economic crisis.

GURS will make adjustments to property values based on data from recent real estate deals so as to make the adjustments realistic, GURS official Martin Smodiš said.

For example, prices for flats in central Ljubljana will be reduced by 16-17%, whereas those of houses in Ljubljana will be cut by up to 24%. Values will also be reduced by around 15% for flats in cities such as Postojna, Nova Gorica and Idrija, while those in Maribor, Novo mesto, Celje and Kranj will be brought down by 10-12%.

One of the biggest reductions will be for office space and shops in larger cities, which are to be reduced by up to 25%. Houses in the area around the town of Jesenice (N) will also see a 25% reduction in value.

The value of building land will also be adjusted, but farmland values will remain unchanged as GURS has not registered a change in prices of 10% or more.

Responding to general complaints about the valuation of properties by GURS, its director Aleš Seliškar told the STA that the data carried by GURS are as good as the information provided by property owners.

While the data for housing units more or less reflect the situation on the ground, data on office space are less reliable because of a lack of accurate information.

Seliškar said that property owners are responsible for ensuring that GURS has accurate data. "Usually in Slovenian people begin to complain only after they receive the bill," he said.

"Those who have not yet taken our data seriously should make sure that they provide accurate data as soon as possible," he told the STA.

He said that he had urged the Finance Ministry to send out estimates of the new real estate tax to property owners with which they would be given a month's time to report changes to GURS.

Morover, Seliškar said that while the system relies on honesty, GURS is trying its best to find cheats. In recent times efforts have been made to discover illegal builds which are not included in the database.

After recent report uncovered an illegal build on the coast, GURS found that the building in question is not included in its database and sent its owner a request to provide data on the building, Seliškar told the STA.

The authority also uses aerial imaging to check for new buildings. In addition, GURS will also launch a project of laser imaging of Slovenia that will allow it establish which buildings have been omitted. The project is expected to be completed by 2014.

The data obtained from the imaging will then also be linked to a building permit database, which will allow authorities to check which buildings are illegal builds.