The body responsible for protecting Scotland’s landscapes and natural beauty warned the £75 million scheme for the former Royal High School would “intrude” into important elements of the Capital’s natural features.
A highly-critical six-page dossier, submitted to councillors and seen by the News, said the proposed hotel scheme would harm public appreciation of Calton Hill.
SNH claims approval for the scheme would also lead to over-development of the hill, which it describes as “a very prominent and iconic landmark within the city”.
Its submission, the second damning verdict on the scheme by a government body, also warns six-storey extensions planned for the A-listed building would also have an adverse effect on views of Arthur’s Seat and Salisbury Crags.
The city council agreed a long-term lease with developers behind the hotel project following a competition to find a new use for the building, which has been lying largely unused since 1968, when the school relocated.
Critics fear Edinburgh will be stripped of its world heritage status if the hotel, which American operator Rosewood will be running, gets the green light.
An alternative proposal, to create a new home for an independent music school and a new concert venue, is being pursued, but has not won the backing of the council.
The SNH intervention will pile further pressure on ministers to call in the development, which has already been heavily criticised by Historic Scotland.
The body has warned the proposed wings for the hotel would “dominate and overwhelm” the building, which dates from 1829.
SNH has now warned the council that the natural heritage interests of the Calton Hill area are of “substantial importance and play a notable role in contributing to the character and identity of the city and the world heritage site”.
SNH has also urged the council to review the impact on the city skyline of both the Royal High scheme and the controversial replacement for the St James Centre, which it approved in the face of huge opposition in June.
In the submission to the council, SNH area manager Iain Rennick said: “Calton Hill is a popular recreational area for city residents and visitors, offering spectacular views of the city centre and other key landscapes within the city and beyond. Its value is widely recognised in the multiple designations that apply to the hill and its immediate surroundings.”
A spokesman for the developers of the hotel scheme, Duddingston House Properties and the Urbanist Group, said: “Every care has been taken to ensure the nature of the latest designs specifically match the contours of the hillside and surrounding environment.
“Many of the features are a direct result of the extensive public consultation process involving dialogue between the developers, local community members and other key stakeholders.”