The cost of clearing the Harris Academy site in Perth Road has topped £2 million.
A report to go before councillors explains that extra spending has included £550,000 for clearing loose furniture, £200,000 on the upgrading of a public footpath and £75,000 on highway works.
The rebuilding of the school, being done through Hub East Central Scotland Ltd, was already expected to cost more than £30 million, with the Scottish Government picking up almost two-thirds of the total and the city council the rest.
City development director Mike Galloway said: “The projectdevelopment fee payable to Hubco for the design and construction of the new Harris Academy amounted to £30,177,930 at financial close.
“However, the site clearance costs were not able to be finalised until the demolition of the existing buildings had been completed and site conditions definitively confirmed.”
Education convener Stewart Hunter said the council had factored in the possibility of increased costs of clearance.
He said: “Whenever you go into an old site and take a building as old as Harris, you are aware there could be issues getting into the building and taking it down.”
Harris pupils and staff have been decanted to the former Rockwell High site for the duration of the project. The replacement school is due to open in autumn 2016.