The firm has agreed to take over another whole floor, or 15,000sq ft, of its Appold Street premises, as well as the remaining 5,000sq ft on the lower ground floor.
The move represents a 40 per cent increase in the size of the firm's City headquarters and will accommodate around 60 extra fee-earners. Watson Farley currently occupies 45,000sq ft over the second, third and fourth floors.
The move will be completed in three phases. The firm took over part of its new floor last week and will get the extra space on the lower ground floor around Christmas. It will move into the remainder of the first floor next spring.
Watson Farley managing partner Michael Greville said of the move: "We're full so we needed more space. I had a choice either to take up the space that became available in our building or to move. It made better business sense to stay in the same building."