Owners of historic buildings will be able to fight demolition rulings that their earthquake-damaged properties are dangerous and insanitary.

Christchurch City Council's agenda for its extraordinary meeting held on Friday revealed how owners might take steps to contest decisions and overturn Government rulings.

Owners can appeal to the Department of Building and Housing's determinations manager, John Gardiner, but his decisions - prolific on the leaky building front - invariably back territorial authorities.

An audit of non-residential buildings by the council revealed how 7600 pre-1976 commercial blocks are earthquake-prone.

Just 220 commercial buildings have been strengthened to varying extents.

"Few would reach the 33 per cent of the current building code now required," the council said ahead of the meeting at which it decided to adopt a policy whereby earthquake-prone buildings would have to aim for a goal of 67 per cent of building code levels rather than the existing 33 per cent.

Holmes Consulting Group's study of the costs of strengthening Christchurch's 490 heritage-listed buildings estimated it would cost at least $169 million to bring these up to the 33 per cent threshold. The 295 unreinforced masonry buildings would cost $137 million. The higher code level would push it even higher.

But seismic strengthening is only one element of the cost.

Strengthening works trigger Building Act requirements to comply with building code provisions on means of escape from fire and installing disabled access, adding a further 20 per cent to 100 per cent increase in costs.

Lost income from the buildings is another factor, the council said.

Upgraded buildings generate more rent, the council said, citing changes after the 2007 Gisborne earthquake.

Richard Peebles, a property developer and investor who owns the former New Zealand Express Co/MLC building at 158-160 Manchester St, is devastated that his Category 1 Historic Places Trust-listed block, built in 1906, was extensively damaged and had been condemned.

Engineers around New Zealand had contacted him and wanted to give their advice.

"But no one is prepared to say it will stand up to the next aftershock so I don't see what choice I have. I've spent most of my working life restoring historic buildings and I won't be there when it comes down. All the businesses within 100m can't operate until the building is either down or safe and to make it safe would take months. I spent all last night trying to work out a way I could make safe it. But no one is going to allow that," he said.

Strengthening work carried out during the 1950s was not enough to withstand the main quake or after-shocks. He backs the council's actions, saying they had provided complete support to building owners.

"They're not responsible for the earthquake," he said.

Windows were smashed to stabilise the block a few days ago by urban search and rescue crews and Peebles said hopes were initially to strengthen the block. But this was not feasible and people were ordered off the site following aftershocks.

The block was fully tenanted and insured but Peebles is concerned about the future of the 18 tenancies in the premises leased for hairdressing, cafe, IT and finance businesses.

Heavy workload
Christchurch quake-prone buildings:
* 7600 pre-1976 commercial blocks.

* 490 heritage-listed blocks.

* 295 unreinforced masonry.

* 29 reinforced concrete.

* 163 timber-frame.

* 220 strengthened to an extent.

- Source: Christchurch City Council