You should check with the town, It may not be accaptable to have access to a roof with no railing, since some people have parties and place a BBQ on the roof, others put pools up there !!
The upstairs apartment needs to have Windows that are up to code in case there is a fire, maybe not access to the roof, now there are different rules in different towns and if a property is owner occupied some rules do not apply but once the property becomes an investment property things may change, sometimes it depends if it is a 2 or 3 family and how many levels of living space you have, some houses need a fire escape depending on the height and number of units.