Virginia Attorney General Mary Sue Terry yesterday dashed the hopes of Loudoun County officials that they could enact a key development measure without the General Assembly's approval.

Terry said in a legal opinion that the county's proposal to encourage developers to provide open space in one section of the county in exchange for permission for greater development in another section must be authorized by the legislature.

Last week, after the House killed legislation for the "transferable development rights," some Loudoun officials expressed hope that they could enact the concept on their own. Terry said "no" to that in an opinion to state Del. Robert T. Andrews, a Republican who represents Fairfax and a portion of eastern Loudoun.

The transferable development rights concept has been a source of bitter controversy on the Loudoun County Board. Proponents say it would allow landowners in the county's largely rural western sections to sell the rights to build on their property to a developer, who could then transfer the building rights to the other sections of the county. In turn, a permanent restriction on development would be placed on the original property.

The measure was lauded by some officials as a way to accommodate Loudoun's building boom while protecting the county's open spaces and rural character.

Terry said that although the Virginia Constitution would appear to permit the transfer concept, special legislation would be required before Loudoun could use it in its zoning regulations.

State law permits localities to encourage the preservation of agricultural and forest lands, she noted, but these measures have legislative approval. The proposed Loudoun ordinance is a means of achieving the same end but has not been approved "expressly or by necessary implication" in any state law.

Terry's decision may cast doubt on at least one aspect of Ashburn Village, a large residential and commercial development in eastern Loudoun, which the Loudoun Board approved in December. The county allowed the Ashburn developer to build at a higher level of density because he had purchased development rights from a farmer in western Loudoun for more than $500,000.

Loudoun Supervisor Frank Raflo, an advocate of transferable development rights, said yesterday he still supports the concept and believes the Ashburn project, which calls for 5,000 houses, will not be affected. Raflo declined further comment on Terry's opinion.