Council bosses are investigating an alleged breach of planning rules by Taymouth Castle developers.
Protest group Protect Loch Tay claims crews are digging deeper than they are allowed at a nearby “borrow pit”.
Taymouth Castle chiefs have permission to extract sand and gravel to a depth of 4m from the site at Inchadney.
But opponents say they have photos which show the works have gone beyond those limits.
It comes just weeks after an application to extend the borrow pit was withdrawn.
Perth and Kinross Council has confirmed its Planning Enforcement Team is investigating.
A spokesperson for Taymouth Estate told The Courier if any breach has occurred it will have been “isolated and unintentional”.
The probe has been launched as work continues on the multimillion-pound transformation of Taymouth Castle and estate near Kenmore.
Discovery Land Company plans to build almost 150 luxury homes, costing £4m and upwards, around a private clubhouse and golf course.
Taymouth Castle developers cooperating with planning probe
The Inchadney borrow pit was created to supply materials for groundworks on the Taymouth estate.
It was approved after the developers said it would reduce lorry movements to and from quarries elsewhere.
The planning permission states that extraction should be kept to approximately 4m deep.
Planning officers are now looking to see whether the developers have gone too far.
A Perth and Kinross Council spokesperson said: “We have been informed of this alleged breach of planning control.
“The Planning Enforcement Team are currently investigating this and seeking to resolve the matter.”
Taymouth Estate says it is co-operating with the council.
“We take our duty to preserve and protect historical sites seriously and work within any planning guidelines,” said a spokesperson.
“If any work is done without following the proper process or in some way goes beyond the planning conditions, it is isolated and unintentional.
“We are collaborating closely with the Perth and Kinross planning department and our local contractors to achieve our goal of 100% compliance.”
‘Regret’ at previous unapproved works
Taymouth Castle developers were investigated for an earlier planning breach last year.
They spoke of their “regret” after work started on the site of a proposed foul water treatment plant before it was given planning approval.
Protect Loch Tay was formed in 2023 to draw attention to Discovery Land Company’s “overdevelopment” at Kenmore.
Its online petition has attracted more than 160,000 names.
Regarding the current investigation, the group said: “We hope Perth and Kinross Council do not continue in their previous manner and just give retrospective consent.
“There must be consequences for their continued breaches of planning regulations which discourage this behaviour going forward.”
Taymouth Castle project forecast to create hundreds of jobs
Discovery Land Company, fronted by US tycoon Mike Meldman, bought the Taymouth Castle estate five years ago
The firm specialises in members-only “worlds”, featuring luxury residential homes in private club communities.
The owners have so far spent more than £100 million transforming the landmark into a private members’ clubhouse.
The company has also acquired a number of properties in nearby Kenmore, including the village shop and Kenmore Hotel.
The project is due for completion by the end of 2029 and is forecast to provide around 250 full-time jobs.
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