I am fitting a shower tray and want to keep it as low as possible (on ground floor shower room). If I fit the shower tray to the floor and have the trap under the floor is this ok or is there times access will be required to the trap?
Depends, if you obtain and fit a top down accessed trap then that will allow cleaning from within the shower. Only access needed then would be if there was a problem with the pipework. Good pipe planning should avoid any issue with that.