It is the latest in a spate of hires for the London headquartered firm, including Bird & Bird construction and engineering partner Colin Hall in April, and Dawsons property partner Simon Livesy in May.
FFW senior associate Lindsay Garratt, who trained under Cooksley, will also join the firm.
The trio have worked together for more than a decade, having joined FFW in April 2009 from Bevan Brittan.
Cooksley, a planning and housing specialist, trained at Winckworth Sherwood before moving to Freshfields Bruckhaus Deringer and on to Bevan Brittan. She will head the firm’s national planning team, which includes partners Sara Hanrahan and Anjana Ghosh.
Managing partner Roger Fitton said: “I am delighted that Karen, Colette and Lindsay will be joining our planning team. They are well known and well respected in many of the areas that the Firm already practices in and will add considerable weight and depth to those areas.”