AS 1657 – 4.1 WIDTH AND ANGLE OF SLOPE Stairways shall be not less than 600 mm wide measured between the inside edges of the handrails.
I’d also carefully what happens if (when) the chair lift fails. Will there still be 600mm of clearance for other residents if the chair lift fails while it is part way between floors? Reading some online rants about these things, it could potentially be stuck there for weeks on end.
What happens if there is a fire and it is stuck or if the owner summons it so that they can escape, but the fire causes the power to fail and now the chair is blocking the exit for everyone – including the user with their limited mobility?