THINK TWICE before you interfere with those trees cutting out the light from the bottom of your garden. They could be worth thousands of pounds.

Researchers from the University of Newcastle claim to have finally proved what estate agents have always known - a spot of greenery does wonders for property prices. Living next to a wood adds up to 7 per cent to a house's value, the university's countryside change unit said.

The unit has also succeeded in putting a pounds sign on what many consider to be priceless - the beauties of the English countryside. It says that the Yorkshire Dales are worth pounds 42m. With more hay meadows and better-maintained drystone walls, their value would be even higher.

The findings of the countryside change programme, backed by the Economic and Social Research Council, will be launched at a seminar at the Royal Society of Arts in London today. They represent the latest attempt by environmental economists to meet the free market on its own ground, by demonstrating that everything has its price.

Philip Lowe, Professor of Rural Economy at Newcastle, said some might find the idea distasteful. 'The fact remains that if you label something as priceless it will be given zero value because decision makers refuse to give it infinite value. The things that we price from the countryside no one any longer values . . . while the things that we value don't have a price.'

Dr Ken Willis, a director of the unit, said more money should be spent on what people really value in the countryside, such as landscape and wildlife. He argues that things are assigned an 'implicit' monetary value, for example in the level of government grant to such bodies as the Forestry and Countryside commissions.

The unit calculates that pounds 900m should be added to the national income accounts to take account of the landscape and amenity benefits of agriculture and forestry. It says people would be prepared to pay pounds 42m to keep the Dales National Park unchanged. However, the annual recreational value of forestry is put at pounds 8.7m, little more than its pounds 8.5m government amenity.