(Reuters) - Homeowners should brace for an additional 20 percent drop in the average value of their homes in 2009 as a vicious recession threatens to slash tumbling prices even further, data from the property derivatives market shows.

According to the latest swap pricing, average house prices are expected to fall by almost 30 percent between December 2008 and December 2010, indicating that the worst of Britain's housing slump is far from over.

The projected 2009 fall is almost double the forecasts for UK house price depreciation from the RICS, and house price index compilers Rightmove and Hometrack.

Average house prices, as measured by the non-seasonally adjusted Halifax house price index, have already sunk to 162,848 pounds in November from a peak of 201,081 pounds in August 2007.

Halifax posted an 18.9 percent fall in UK house prices in 2008, while rival Nationwide recorded a 15.9 percent fall over the same period. Both groups originally predicted static house prices in 2008 and have declined to make a 2009 forecast.

The young and relatively illiquid over-the-counter market provides investors with an opportunity to increase or hedge exposure to the UK housing market synthetically -- in a cheaper and more efficient way than buying or selling bricks and mortar.

(Reporting by Sinead Cruise; editing by John Stonestreet)