A MAJOR survey of hundreds of listed buildings in the North York Moors National Park is to be carried out over the next seven years to clamp down on unauthorised alterations.

Beth Dexter-Smith, the park's new building conservation officer, says: "The built landscape of the park is just as important as the natural environment.

"It is important to protect listed buildings because they are a valuable and finite asset."

She added: "One of the authority's main concerns is the number of small changes that are being made to listed buildings without consent. The replacement of guttering, windows and roof tiles with unsuitable materials has become a widespread problem.

"Often, owners make these changes because they do not realise that planning consent is necessary. But these modifications do have an incremental effect.

"The use of plastic guttering, plastic windows or concrete roof tiles gradually erodes the character and historic qualities of a building."

Following an inspection, if a problem is identified, a member of the planning team will notify the owner and generous time will be given to comply with the law, she said.

Meanwhile, owners of historic buildings are being urged to use lime mortar rather than cement.

"Over the last 20 years, it has become clear that cement-rich mortars are unsuitable for use in historic buildings and lime is enjoying a revival," said Mrs Dexter-Smith.

Cement, she said, was hard, brittle and impervious to damp, while lime was softer and more flexible.

Ms Dexter-Smith said the park is to hold a "lime day" next spring to show builders and property owners the benefits of using lime. Further details can be obtained from the national park office on (01439) 770657.

A survey by English Heritage of 21 local planning authorities found that the North York Moors National Park Authority had:

* the highest number of listed buildings; the highest number of Scheduled Ancient Monuments;

* The highest number of individual grants available for stakeholders to apply for;

* The fourth highest budget available for

investment in the historic environment.

Updated: 09:26 Monday, December 22, 2003