Councillors in Salford, Greater Manchester, ruled yesterday that the block of five £300,000 apartments has broken its original planning permission.
The building is 35cm too high and 45cm too wide on one side because of a window. Stringent planning restrictions were imposed on the original development, called Bridgewater Court, because the building is in Worsley, a conservation area.
Salford City Council is now planning to issue an enforcement order that could see the scheme demolished.
The developers will be forced to either remove sections of the roof and window to bring it in line with planning regulations - or knock down the entire development.
Councillor Derek Antrobus, Salford council's lead member for planning, said yesterday: `The developer will have to change the building to fit in with the original plans.
'We are not going to be afraid to make tough decisions like this. No developer will be able to flout the rules like this and get away with it.
'They should simply stick to the original plans that were approved by the city council,' he added.
The two-bedroom apartments have already been extensively advertised. Simon Richardson, a lawyer and planning consultant acting for the developer, Urban Guides, said they would appeal any enforcement notice and hope to come to a compromise with the council.
He added: `The councillors seem to want to make a bit of a fuss about this. But I am optimistic sense will prevail.'