If the intention is to just use the roof space in the house as storage then it is not necessary to install a staircase; instead an access hatch and ladder can be used.  A hatch can be purchased as either a package with a loft ladder or separately. They are usually supplied with a white, textured finish, spring-loaded catches and are insulated and sealed. Pre-manufactured units can also be supplied with their own frames which will fit neatly into the enlarged opening you have created and are easily and quickly fitted. They usually require no finishing once they are installed. Usually opened with a pole pre-manufactured loft hatches are also available with locking catches. So, if storing valuable items in the loft conversion, using a lock on the hatch is an excellent added security precaution.