Do the same staircase rules apply in a flat?
Yes, the same rules apply to stairs in a flat; nothing is different.
Related questions
In a flat, if part of the shape is cut by the stairs or lift, how is that part treated?
The flat is the part you actually enter and use. Once inside, your movement and work happen only in that area, so the part you cannot access, taken by the stairs, lift or the neighbouring flat, is a cut in your flat.
In a flat, if the stairs are in the anti-clockwise direction of the house, is it a vedh?
It is a dosh, not a vedh. Stairs carry the geopathic currents from below up the building; a current moving clockwise grows while one moving anti-clockwise is lost, just as spinning clockwise and anti-clockwise feel very different.
A house has its entry in the Agni kon (South-East), an underground water tank in the South-East with the stairs over it, and the slope running to the South. The family fights constantly. What is the remedy?
Either leave the house or treat it: close the South-East underground water tank immediately and correct the slope. The staircase can stay. An overhead tank in the South-East is acceptable.
A relative's flat had its door at E1. After buying the adjoining flat and removing the common wall the entrance came onto N4, and soon their ancestral land was acquired for a bullet-train project and they received crores. Is this the N4 effect?
Yes, this is chhappar phaad wealth, which comes as soon as N4 opens. E3, N4 and W4 are the three vipul sampada dwar (abundant-wealth doors). Opening them virtually gives perhaps 1 percent of the actual result, so do not believe the heavy promotion of virtual doors; it is the actual opening that brings the result.
Are marma and ati marma applicable in flat vastu?
Yes. The ati marma of a flat should also be found and kept free, just as in a house or plot.
Are stairs made of mixed iron and plywood acceptable in the North?
Stairs should not be built in the North at all; if they must be, use wood. Many showrooms with wooden double-height staircases are found to do very well, because wood represents Mercury just as stairs and business do, so wooden stairs boost intelligence and keep business running.