In the building code, stairs are treated as a part of a means of egress. There are certain requirements that they need to meet, such as a maximum riser height, minimum depth of treads, minimum width of stairs, landings at top and bottom, hand rails, etc., but if the stairs meet the requirements, they are considered to be an acceptable part of a means of egress. I think that if the stairs in question do not meet the building code requirements for egress stairs, then they could be considered to be an obstruction.