Extension adjectives are listed in this post. Each word below can often be found in front of the noun extension in the same sentence. This reference page can help answer the question what are some adjectives commonly used for describing EXTENSION.
active, actual, additional, agricultural, anterior, apparent, appropriate, arbitrary, artificial, automatic, back, backward, boundless, broad, certain, common, complete, consequent, considerable, constant, continual, continued, continuous, cooperative, corresponding, dangerous, day, definite, different, digit, dimensional, direct, downward, early
eastern, eastward, easy, effective, enormous, entire, excessive, exe, extraordinary, extreme, file, filename, final, finite, first, foot, forced, formal, former, forward, free, fresh, full, further, future, general, geographical, gradual, great, greater, greatest, hip, horizontal, html
immediate, immense, important, indefinite, inevitable, infinite, interesting, intracranial, joint, knee, large, larger, last, later, lateral, legitimate, less, liberal, like, limited, limitless, linear, little, local, logical, long, longitudinal, lumbar, major, marked, massive, maximum, mere, metaphorical
moderate, modern, month, more, much, natural, necessary, neck, new, next, normal, northern, obvious, only, optional, orange, other, outward, own, particular, passive, permanent, physical, possible, posterior, powerful, practical, present, probable, prodigious, progressive, proper, proposed, proximal
pure, radical, rapid, real, rear, reasonable, recent, relative, remarkable, same, second, semantic, short, significant, similar, simple, single, slight, slow, small, southern, southward, spatial, special, specific, steady, straightforward, subsequent, successful, such, sudden, superficial, temporal, temporary
term, terminal, territorial, theoretical, total, true, tumor, ultimate, undue, unique, universal, university, unlimited, unprecedented, unwarrantable, unwarranted, upward, useful, utmost, valuable, vast, vertical, very, visible, western, whole, wide, wider, widest, wonderful, wrist, year
Hope this word list had the adjective used with extension you were looking for. Additional describing words / adjectives that describe / adjectives of various nouns can be found in the other pages on this website.