Annex(e)

The spelling of annex can be troublesome. OED records that both spellings, annex and annexe, exist - when annex(e) is used as a noun. It only records the shorter form when '(to) annex' is used as a verb.

When '(an) annex(e)' is used as a noun, it is best spelled with only one '-e-'- in the middle, not at the end. Although modern practice is variable, AWE recommends all writers to use the spelling annex at all times - except when it means "a supplementary building designed to supply extra accommodation for some special purpose; a wing" (OED), which derives this use "From the mod[ern] [sc. nineteenth century] Fr[ench] annexe, as applied to additional parts of an exhibition building".

Don't be misled by the fact that endings suffixed to the word are written with an '-e-': this is the normal way of adding them to words that end with a sibilant like '-x-': many educational establishments have annexes, where the singular is annexe; but documents may also have several Annexes - where the singular is best written Annex.