The officer, Martin Higgins, had previously opposed the plan arguing that the area within the proposed extension does not warrant inclusion.
However, lobbying by the Haslemere Society has brought about a U-turn – bringing it a step closer to being approved.
On Tuesday, Waverley Borough Council's executive considered the proposals to extend the conservation area to include much of the Courts Mount Road area as well as Sandrock and a portion of Shepherd's Hill.
The proposals were put forward by the Haslemere Society in a bid to protect the area from inappropriate development.
However, as previously reported by The Herald, the society was dismayed when it learned that the Surrey County Council officer opposed the plans arguing that the area did have sufficient enough "special character".
This prompted the society into action and members met with Mr Higgins to show the officer its research in a bid to change his mind.
The society's actions worked and on Tuesday, members of Waverley Borough Council heard that the historic buildings officer had altered his views.
In his new comments, Mr Higgins divided the proposed extension into three distinct areas and said that there is a case for including all of them.
For the Shepherds Hill and Sandrock area, the officer said that the society has provided extra evidence of the cottage hospital in the area as well as the use of the common for stoneworking, water supply and recreation.
"These associations, but in particular the siting of the cottage hospital on the common, is consider by (the officer) to form an appropriate basis for conservation area status," the council's updated report said.
The officer then turned his attention to the housing abutting Sandrock and said that 18th and 19th century maps show that the route may have led to the west side of the valley and plans a part in defining the edge of Sandrock Common.
This, the officer now argues, forms a justification for inclusion within the extended conservation area.
The last area addressed is the properties fronting Courts Mount Road Road and Courts Hill Road where new development has taken place in recent years. The society provided information that the area demonstrates "a continuous line from the Tudor period to the present day".
Councillors heard that the officer "fully accepts that this argument can be made". News of the change of heart was welcomed by councillors at the meeting.
"I am a lot happier that Surrey County Council are starting to listen to the views of the local people and not going their own way," Haslemere councillor John Robini said.
"It is time that we did listen to the local people."
Robert Serman, from the Haslemere Society, was also delighted with the U-turn which came after the society met with Mr Higgins and showed him its research.
"We are very pleased with his comments," he said.
MP Jeremy Hunt, who has spoken out against inappropriate development in Haslemere, was also "delighted" with the news.
"We need to do all we can to preserve Haslemere's history and heritage," he said.
Members of Waverley's executive agreed to support the proposal and recommend that it be given the go-ahead.
However, critics of the extension plans have vowed to continue the fight despite the county council's changing attitude.
Courts Mount Road resident Darren Ellis, who has submitted plans to replace his home with 12 flats after it was surrounded by other similar developments, described the reverse as "both surprising and disappointing".
"They have obviously come under pressure from the Haslemere Society and Action Group, and probably the borough council too," he said.
"The decision to extend the conservation area is laughable given the developments that have taken place around our plot in particular.
"By anyone's imagination it cannot be considered a conservation area."
The final decision will be taken at a full Waverley Borough Council meeting later this year.