To the casual observer, the two-level office block opposite Flagstaff Gardens in King Street wouldn't bear a second glance.

It is a squat brown, steel-ribbed building designed by one of Victoria's leading 20th-century architecture firms, Yuncken Freeman, which is responsible for some of the city's best 1970s office buildings, including BHP House and 1 Treasury Place.

The brown, brick offices on the corner of Batman and King streets, West Melbourne. Credit:Penny Stephens

But Flagstaff House, on the corner of Batman and King streets in West Melbourne, is a modernist heritage building and a battle is brewing to save it, led by Melbourne City Council and heritage activists.

The developer behind a proposal for the site plans to maintain the exterior of the 1968 building, just not well enough, the heritage experts say.