You could carry the step on and square it off to the doorway, so that you actually have the first step up out of the doorway. If the step is too high then you can split it into two so you have two square and you still get rid of the dubious triangle. I also saw a picture on Pinterest recently where the bottom step of someone's stairs had been made into a draw that slides out and stores shoes. If you had a large square step then this would be big enough to carry that off so you can make use of the space as well.

I won't lie, that staircase looks awfully steep too!

I guess it all depends on your budget, but I would most certainly look at moving the doorway away from the stair allowing you to build back that step. mrsmcee74 is right, it's an accident waiting to happen!

If budget stretches & further to moving the doorway, I would look into getting a new staircase made and installed, with correctly calculated and equal step heights. That bottom step looks very high and I bet it feels weird when walking down!

You could look at making a feature of the step by making it larger and sweeping it out into a curve - you have enough room to take this into the hallway - with a well placed selection of photos/pictures maybe starting at the end of the where the step meets the wall and moving up the stairs and a radiator cover it could look very nice and a deliberate design feature rather what it looks like now - a functional and badly done step!

i'd be considering a new staircase, that bottom step looks very high, have you measured it compared to the other steps. (height = riser, depth = tread)

Nina, let us have the floor plan, there is nothing much we can see from this photo other than a very steep stair with no space left or right where you could fit a bend. What is behind that stair ? Is it an empty space or it's all brick ? Could you manage to replace this one with a airy stair made of steps only ? (with no counter-steps) so that it doesn't look cramped anymore ?

Looking at the picture it appears that the whole staircase and wall are both skewed thus causing the very awkward angles involved. Clearly a major rebuild would be the perfect answer but I have a simple solution that will cost little and might be a temporary fix.

Simply make the bottom step into a two thirds wide step (measured across the stair width) giving the full tread depth but only 66% of the width. To reduce the step height, make it two steps but you would need to be careful not to create a trip hazard from the downstairs doorway. You might consider adding a short extra handrail on the stair wall to the left looking up the flight.

In fact just this week I saw a house with a very steep loft conversion staircase that had the full flight done in half steps so that you climbed each with a left/right/left/right step movement and it was called a "Hit and miss" staircase. I am not saying it looked a good idea as regards safety etc. but at least it got a staircase into a position where a conventional design would be even worse!

As I said....a full rebuild is the best solution but the above might help short term and be better than trying to cope with that very narrow/disappearing first tread.

Previous owners got used to it, huh? After falling and tripping how many times? I'd hate to have to carry anything up or down. Yes, do something - I have no idea, but there are some great suggestions here. I just wanted to add that the stair storage picture was also in an article on Houzz
https://www.houzz.com.au/discussions/this-or-that-under-stair-shoe-storage-dsvw-vd~3148654?n=6

so when you do get to the rebuild the lot stage, it's something you can remember for inspiration.

I would do as others have suggested - move the door. Just needs to be enough to allow the step and architrave surrounding door to fit.