Dibb Lupton Alsop is to move its four London offices into one new building.
The firm has signed a new lease on a building in Noble Street, directly opposite its current main London office at 125 London Wall.
The move gives the firm enough space to double the number of its lawyers working in London and help Dibbs towards its stated goal of becoming a top 10 City firm.
Dibbs currently has around 240 lawyers working in the City and the new office will accommodate just under 500.
Property partners Philip Perry and Richard Parker negotiated the 20-year lease which will initially cost the firm u4m a year in rent. The agents for 3 Noble Street were Healey & Baker and Richard Ellis St Quintin and Savills acted for Dibbs.
London managing partner Paul Nicholls says: "Philip and Richard have engineered a superb deal for the firm and have delivered offices that will enable us to fulfil our vision to be a dominant legal force in the City."
The move will happen early next year and the Chase Manhattan bank will take over the firm's London Wall premises for the remaining 10 years of its lease.