Expensive Living accomdation benefit

I have aquestion regarding what cost of providing accomdation to use in the subsequent years of assesmment like if the employee first moved in 2009 and the company purchased the accomd. in let say in 2006 it is obvious that this period is below 6 years then we use the cost but what about subsequent years when we reach the 6 years limit we turn her eto market value or we keep jsut using the cost as when he first moved in the time was below 6 year..please advice