The deal, which has now been agreed by both the US firm and the UK parent Dyer, will represent the end of an era in the hospital-design market.
The name Anshen Dyer will go and most of operation will simply become the British wing of the American giant. Dyer will revert to being an independent business.
While no real reasons have been given for the move, observers have pointed to this year's AJ100, which saw the practice slip 21 places, with a drop in the number architects employed from 103 to 54.
In an official statement, the explanation for the changes is so that the company can 'reflect the growth of the marketplace and its future ambitions'.
The changes will see Dale Sinclair, who has been on a long-term secondment to the firm as its MD, revert to being a director of Dyer.
Founded 20 years ago, Anshen Dyer has become one of the leading practices specialising in areas such as PFI healthcare work.
Speaking about the changes, Sinclair said: 'Working jointly in practices has allowed this partnership to become the UK market leader in healthcare-design services.
'Once under independent direction we will have the freedom to pursue our unique vision of healthcare and education reform in the UK and abroad.'
Derek Parker, one of the original co-founders of Anshen Dyer, remains chairman of Anshen + Allen's UK operations. Directors John Cooper, David Martin, Jane McElroy, and Felicia Cleper-Borkovi continue to lead the design and project management of the practice. Kieran Breen replaces Sinclair as managing director in London.