Has anyone had an external spiral staircase installed

I know this is a long shot!

We recently bought a fortuna style terraced house and have noticed one or two around the general area who have installed a spiral staircase from the balcony to access the mostly flat roof to turn it into a sun terrace.

In addition to installing the staircase there would be some partitioning and safety walls to be installed.

This is a possible future project so I wondered had anyone had similar work done and what the approximate cost was be so I can file it under "get quotes next year" or "delete from thoughts" before I waste anyones time!

Thanks