At a recent meeting of Architecture + Design Scotland's (A+DS) design review panel, the design watchdog panned the proposals by Edinburgh-based practice Hurd Rolland to extend the mansion, built in 1777.
Among the criticisms levelled at the scheme to increase the size of the Georgian house - now a luxury hotel - were fears the design could slip into 'historical pastiche'.
The report said: 'The current proposals are wholly inappropriate in dealing with or complementing the existing building.
'Extensions to existing buildings should either conform faithfully to the design, craftsmanship and quality of the existing building or alternatively offer a contemporary solution that celebrates and enhances the original architecture.'
It goes on to say: 'Should a contemporary approach be taken, we would encourage the design team to look towards such precedents as RMJM's Tron Theatre extension in Glasgow.'
In response, a spokesman for Hurd Rolland said: 'The design submitted for planning approval evolved from an existing Planning and Listed Building consent for an extension in this location, which closely followed the existing building.
'We have received a number of diverse comments, including those from A+DS, and we are taking account of these in developing a contemporary design solution in consultation with Planning and Historic Scotland, appropriate to the Grade A-listed building.'