Planning/Regs is only really important if you’ll want to sell the house on.
Which unlike a car which you can run into the ground and scrap, is a distinct possibility.
You will (99% sure here) need a building warrant as it will probably involve structural work, as such you would be subject to regs BUT in the case of stairs this can only be a good thing, modern stairs are designed to be a decent size and gradient.
Don’t arse about with non regulation stuff, having been on the recieving end of it myself any sensible buyer (or decent conveyancing lawyer) will be all over that and it’ll cost you to take out indemnity insurance (which in our case was worthless since the council weren’t bothered but the extension was still falling down – once bitten twice shy, dealbreaker next time).