Is the stairwell from the second floor to the third floor in Beijing illegal?

That depends on how the control rules of the original plot are required. If the original building has achieved the ultimate in height control and building area, then rebuilding and adding stairwells on it will naturally break the height control and naturally belong to illegal construction. Any building that is not approved by the Planning Commission or the Land and Resources Bureau is regarded as illegal. Illegal buildings belong to the demolition offices, urban construction departments or comprehensive management offices of townships, towns and sub-district offices at all levels. I suggest you contact the above units to report. The hotlines of local governments are different. You can inquire through 1 14.