Newport City Council says "Newport has 15 conservation areas" and that "some conservation areas in Newport have an Article 4(2) Direction in place, this means that works that were previously allowed now require planning permission". Its own list of what then needs permission for residential properties includes to "build, renew, alter or demolish any part of a gate, fence, porch or wall". The council names a direction at Caerleon, one at Lower Dock Street implemented in April 2002, and Article 4(2) Directions introduced in 2020 at Stow Park and at The Shrubbery.
Newport is in Wales, so the Welsh rules apply and the English Planning Portal does not. Welsh Government guidance says you will need to apply for planning permission if you erect or add to a fence, wall or gate and "it would be over 1 metre high and next to a highway used by vehicles (or the footpath of such a highway); or over 2 metres high elsewhere", or if "your right to put up or alter fences, walls and gates is removed by an article 4 direction or a planning condition", or if "the fence, wall or gate, or any other boundary involved, is in the curtilage of a listed building".
The same guidance says "you will not need to apply for planning permission to take down a fence, wall or gate, or to alter, maintain or improve an existing fence, wall or gate (no matter how high) if you don't increase its height". It adds that "in a conservation area, however, you might need conservation area consent to take down a fence, wall or gate".
In Newport we would explain how you check the conservation boundary, any Article 4 direction and the proposed work before confirming whether permission is needed.