Synonyms for Addition:

n.

bay, brickwork, Totalization, catacombs, ceiling, alcove, balcony, beading, body, cornerstone. accompaniment (noun)
extra, lieutenant, appendage, accompaniment, extension, attachment, appendix, auxiliary, adornment, fixture, companion, consort, second, accessory, adjunct, appurtenance, garnish, addendum, brother, ornament, secondary.
acquisition (noun)
appropriation, gain, collection, annexation, catch, claim, purchase, procurement, assumption.
addition (noun)
accumulation, accession, improver, aggregation, augmentation, enlargement, summation, gain, accrual, increment, plus, acquisition, expansion, attainment, increase.
arithmetical process of augmentation (noun)
summation, Totaling, reckoning.
artifact (noun)
improver.
augmentation (noun)
annex, swelling, amalgamation, inflation.
enclosure (noun)
insert.
increase (noun)
raise.
mathematics (noun)
arithmetic.
process of conjoining, adding (noun)
annexation, attachment, augmentation, enlargement, increasing, extension, accession.
something conjoined to or enlargement of something (noun)
additive, appendix, increment, commission, addendum, extra, dividend, appendage, annex, accessory, bonus, expansion, increase, aggrandizement, raise, accretion, accrual, reinforcement, enhancement, wing, adjunct, gain.

Other synonyms:

bay, alcove, complement, costing, computation, ceiling, Totalization, overkill, cornerstone, catacombs, determination, overcapacity, brickwork, assessment, beading, balcony. overflow, overrun, room, spare, wing, surplus, correction. calculation, count, body. excess. accumulation
accretion.
premium
interest.