(Reuters) - British house prices grew at their slowest rate in a year and a half during the past three months, a property industry body said on Thursday, but a planned cut to property taxes is likely to temper the lull in sales.

The Royal Institute of Chartered Surveyors said its monthly house price index sank to +13 in November, down from +20 in October and its lowest level since May 2013, when Britain's housing market started to pick up strongly.

Economists polled by Reuters had expected the index, which measures surveyors' view of price trends over the preceding three months, to drop less sharply to +17.

Britain's housing market has been slowing since the middle of this year, due to tighter mortgage regulation and annual price rises of more than 10 percent that have far outstripped meager wage growth.

But RICS saw the potential for a boost from tax changes announced last week by finance minister George Osborne which, he said, will result in lower payments of the stamp duty tax for more than 90 percent of home buyers.

"The stamp duty reform could reverse the softer trend in buyer enquiries that has been visible in recent months," said Simon Rubinsohn, chief economist at RICS.

Surveyors on average expected the change to increase transactions by between 2 and 5 percent next year and raise the supply of homes for sale, RICS said.

Forecasters at the government's Office for Budget Responsibility have said they expect the tax changes to increase annual property sales by 1 percent in the long run, as well as push up the pre-tax selling price of many homes.