Have an old staircase down to the basement, planning to have a staircase beside it where you can go up to a new floor. Probably need to widen the joists properly and open a shaft all the way from the basement, and remake - do it right.
A staircase in a single-family home that serves as an escape route must never be narrower than 90 cm. If you have a staircase that turns 180 degrees via a landing, the width may often need to be increased to 110 cm if you want to be able to carry furniture, for example.
Sofas and spring beds are usually the challenge. A straight staircase can probably handle most things. The problems start when it turns. 180° can be tricky. I've checked the figures in one of my old handbooks which is very reliable.
personally, one has to search the internet, and try to filter facts and impressions to the best of one's ability.The best is to search in antiquarian bookstores. The handbook "Bygg" is available in six volumes and multiple editions. Volumes 4 and 5 are probably the most interesting for you. A bit outdated in some respects, but still relevant in others. Especially when you have an older house, it can be valuable.
Few things are as well-studied as the minimum dimensions for staircases. Usually, this refers to the transport of a stretcher. The minimum measurement for a staircase with two flights and a landing is typically given as: stairwell width 240 cm, landing depth: 130 cm. If you make the landing deeper, the staircase width can be reduced. The presence of handrails/railings also affects transport possibilities. If you are sensible, you make these detachable for special occasions. If you want to examine this yourself, just make scale paper models and test. Normal mattress size is rather 105x210 cm.