Provisions for those people who may need a ramp should always be considered in the first instance, whether retrospective or new-build.
However, there are many existing buildings where a ramp is just not feasible due to the existing configuration of the exit - for example, there may not be room to fit it in.
In such cases some other provision should be made to ensure that all those who may need to use an exit are able to do so safely. And an example here might be the allocation of a member of staff to assist people at a certain exit during an evacuation.
Of course, it may be the case that an exit is never likely to have a disabled person making his or her escape through it (for example, a plant area where no unique facility exists requiring access for all). If that is the case then there is no need for a ramped egress route.
Stu