bp wrote:
Lifting up the carpet I could see that the step (the bit of wood that
you stand on when climbing) is totally worn out. I would say the
stairs are the original ones from when the house was built.

[snip]

Is there a way where just the steps can be replaced?

Yes it's possible, but means virtually dismantling the existing
staircase to replace the treads. That's certainly the best way.

What the previous owner did (fixing wood on top of the worn steps) -
is that OK to do. I could use better wood (possible ply etc) and fix
onto all steps. the steps would all be the same. Is this OK in terms
of safety etc.

There's nothing really wrong with doing that, but using chipboard to do
the job is totally, utterly stupid as it's the flimsiest stuff there is.
Having said that, bear in mind that the height of the top step will
still end up lower than the others, and the bottom one will be higher.
Might not be considered safe? You need to use the hardest but thinnest
wood possible. Thinks... I wonder if you could clad them in 6mm
plywood, and then cover that with a L-shaped plate of thin aluminium to
protect the leading edge?

Just to add I would not be doing this I would be instructing a
carpenter. I just want to know what my options are and whats a good
plan of attack

I should ask for quotes for doing it both ways, compare them and decide
whether the cost difference is worth it for getting the job done properly.

David