Downing has hired Glenn Howells Architects to design a mixed-use tower of up to 20-storeys to extend The Capital in Liverpool city centre.
If built, the scheme would add a tower to the side of the Capital facing The Strand and the River Mersey.
Pending a successful feasibility study, a planning application could be submitted in spring 2008 for a mix of offices, apartments and a hotel.
Paul Houghton, development director at Downing, said: "Glenn Howells put forward a stunning proposal, which will enhance are ongoing plans to remodel the Capital as a modern, cosmopolitan business hub.
"We have always believed that the Capital offers massive development potential. It is a quality building in a superb location at the heart of the business district and overlooking the waterfront. We are working closely with the city council to bring forward a striking mixed-use scheme."
Downing acquired the 600,000 sq ft building for £51m from Royal & SunAlliance in March 2006. An initial phase of refurbishment of existing office suites and installation of a new grand reception is nearing completion.
A second phase of work was granted planning approval in June. The £13m project will add 18,000 sq ft of office space to the building via glazed extensions to the eighth and ninth floors and is set for completion by 2009.
The Capital is the front-runner to secure the coveted letting to the new super-agency being created by the merger of three of Liverpool's regeneration organisations.
GVA Grimley and Keppie Massie are letting agents on The Capital.