I’m renovating a house and the ground floor is seriously on the piss. As far as i can work out it was partly built like that (~1935) and partly moved a bit / joists bowed a little etc.

Anyway, working from the highest point and replacing rotten joists / packing out the plate on the sleeper wall is all fine until I get near the stairs. By the time I level that out and put a chipboard on then the finished flooring, the bottom step on the stairs is going to be something like 50 – 55mm lower in height than the rest of the stairs.

I’m sure I’ll notice, but will it really be an issue? Any thoughts?

Crikey!
Just been and checked
My bottom step is lower than the rest too!

It gets worse: The eleventh is lower than the twelfth, the tenth lower than the eleventh, the ninth lower than the tenth…………..

but will it really be an issue?

imo yes. But you’ll get used to it. Visitors might not though.

btw, if you’re wondering in which way will it be an issue, think of how awkward it is to walk up or down the first and last bits of an escalator which has broken down – all risers should be of an equal height.