Disabled Access

Access

We recently rented a first floor flat in a building that is an old house converted into 6 flats. There is stepped access to the building and the flat in question is one floor up in the building. The tenant within a month of moving in has requested we fit a ramp to provide disabled access to the building. This was not mentioned when they viewed the property not to mention that they have taken a first floor flat which has far more stairs to get to from the ground floor than the number of stairs to access the building. Surely this is something that should have been requested before signing for the flat. Are we obliged to provide disabled access? and if yes who is responsible for the cost.