Yorkshire House on Rougier Street was built in 1962 and refurbished in 2000 before being sold by Aviva.
North Star, with funding from Lothbury Investment Management, set out to convert the building for occupancy by Malmaison.
North Star is acting as main contractor, working with Dudleys, DLG Architects and Martin Design Associated, to deliver the project.
A spokesman said: "Dudleys delivered full structural engineering consultancy services for widespread alternations to the existing concrete frame of the 29,000 sq ft landmark building, a rear extension, as well an upwards extension for an impressive new Sky Bar on the roof, affording spectacular views of the city walls and York Minster."
Paul Brownlow, Director at Dudleys said, “The conversion of Yorkshire House was a sensitive project given its prominence and tight city centre location.
"Balancing the structural integrity of the building with the client requirements for open plan rooms was a challenge as well as working within a flood risk strategy. We delivered a solution to open the spaces from boxed in offices with minimised structural intervention using lightweight techniques.
“A particularly impressive part of the project was the sky bar extension where the floor was recessed to lower the standing area for bar staff so that clients can look over them towards the Minster. It is great to see this building revitalised with a stunning internal reconfiguration and in operation by such an aspirational brand.”
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