Do I need planning permission to put an external staircase & railing to an existing flat roof

Planning permission *and* Building Regulations.

Planning will involve neighbour notification and overlooking is likely to be a significant ground for objection (unless you have a big garden or are somewhere rural).

And a "former conservatory" roof is most unlikely to meet the strength requirements for a walkable-on area.

Owain

Yes I see applications for this sort of thing all the time on council lists. I do not know how many are approved though, I'd not be interested unless they were next door. There have been some horror stories in the paper of people making roof gardens so the bedroom looks out on nice foliage, then they have to figure out how to water the plants and indeed on how to get the planters, etc up there, and finally the day there is a huge downpour and it sags under the weight and the nice dry inside turns into a kind of unofficial wet room with drips and bits of ceiling raining down on the posh kitchen inside.I do wonder at the stupidity of people sometimes. I mean a cubic metre of water weighs a ton. Literally. Wet earth? Yuck. Brian