1) Added 2) Auxiliary 3) Extra 4) Extramural 5) For an extra charge 6) Further 7) Further or added 8) More 9) Not full member 10) Not on the floor model 11) Optional 12) Other 13) Second 14) Secondary 15) Spare 16) Superfluous 17) Supplementary Found on https://www.crosswordclues.com/clue/additional
[Adjective] Something which is added or extra. Example: When they booked the holiday, they asked if there were any additional costs.
Found on http://www.bbc.co.uk/skillswise/glossary/