The Edinburgh-based bank's latest legal move follows a successful reversal of its Court of Session defeat at the hands of now bankrupt developer Derek Carlyle, where three Court of Appeal judges ruled that RBS was bound only by what was contained in its written contract.
In a Court of Session judgement in June this year, Lord Malcolm commented that the bank's recovery action against developers Ian McDonald and Joe O'Donnell was "a case study of the causes and consequences of the property crash", and ruled against the bank.
RBS had backed the developers with a £1.65 million loan in 2007 to buy a piece of land in Greenock - which was sold off three years later for just £65,000.
The judgement revealed that following the crash, the RBS credit division instructed that all property loans needed at least 30% security, and that the critical valuations of sites appeared to vary wildly depending on which firm of surveyors was instructed.
The two developers were persuaded to sign a £300,000 personal guarantee in March 2009 on the basis of an updated site valuation produced by leading Scottish surveyors Ryden, which the judge said appeared "designed to ensure that the end result met the figure previously promised" in discussions between bank and surveyor.
Lord Malcolm said the borrowers would not have signed the guarantee "if they had known that the Ryden revaluation could not be relied upon as a professional opinion from a large and respected firm of surveyors".
He commented: "It would appear that vastly differing valuations of the site were obtainable depending upon whatever assumptions the valuer was asked to, or chose to adopt."
Ryden however demonstrated that it had warned the bank that it had been instructed to produce only a "desktop revaluation" for a £500 fee, not a full £2500 valuation.
In its grounds of appeal, RBS says its valuations were not misrepresentations but "simply passing on information...provided to the pursuer by professional advisers".
The bank denies that it had a duty to alert the developers to a letter from Ryden warning that its valuation was "for indicative purposes only", and denies that the defenders suffered any loss as a result.
The banking giant faces paying the defendants' costs and also a counter-claim for interest.