He said that all work had been stopped on the affected areas of the site until they could be made safe, but that workers were back in unaffected areas.
He said: "Until we get the results of the full structural review we will not know what the extent and cost of the damage is."
The spokesperson said it was too early to assess how long the construction programme would be delayed by the fire.
He added that a specialist cleaning company had been employed to clean up smoke damage on the site.
It took 40 fire fighters about half an hour to control the blaze, and roads in the neighbouring town of Otley had to be cordoned off.
Police are treating the blaze as suspicious and the site is being examined by police and fire investigators.
It is understood that the police have already arrested a 15-year-old girl in connection with the incident.