Archaeological finds, possibly from the English Civil War, have delayed the development of one of Bristol's most well-known derelict sites.
The building has been empty since 1986
The rare defence works on the Westmoreland House site will have to be recorded and removed if they are confirmed as genuine.
The finds have put back Comer Homes' plans for 200 flats and a complex for arts group Kumba on the site.
The council said it had received a planning application last October.
The 1960s office block dominates the 1.26-acre site, which Comer bought in 1989.
Since then, it has had problems with fly-tipping and graffiti, a spokesman said.
Earlier in May, a man was seriously injured after falling four floors in the building.
Comer said it would be on-site within three months of getting planning permission.