Dreamt up by Donald Insall Associates, the project will create 18 luxury apartments in a roof-level extension overlooking the site of Will Alsop's failed Fourth Grace project.
The floor replaces a post-war solid Portland-stone attic - itself a replacement for a bomb-damaged mansard roof - and will form part of a larger scheme to convert the 1907 office building into a mixed-use development.
The proposals, which have been recommended for approval by council officials, will go before the city's planning committee next Tuesday (11 October) and have already been given the thumbs-up by English Heritage.
As well as opening up the building's huge dome to the public, the project will also create new shops and restaurants at a refurbished lower-ground floor.
In addition, the car park facing the Mersey will be replaced by a new sheltered public area sunk behind the existing wall.
According to a spokesman for the practice: 'The cars will be relegated to an underground car park and the 'piazza' will be served by the new café and retail spaces within the building.'