Up to £24m extra could be spent revamping Norfolk County Council's headquarters - at a time when people are facing a council tax hike and cuts loom which would hit the vulnerable.
Opposition councillors criticised the move, which would take the total spent on County Hall since 2013 to almost £60m.
A revamp of the 1960s-built headquarters began in 2012, after blocks of masonry fell.
A £22m contract was awarded to RG Carter, but an extra £10m was added barely a year later, because the original programme assumed existing insultation, heating and lighting would be replaced with similar systems.
A further £7m was added in 2017 to revamp the basement and lower ground floor, plus the north wing. However, it has emerged the bill to do that now stands at between £22m and £24m.
The council says it needs to accommodate staff leaving other locations.
A spokesman said: 'One of our long-term aspirations is to have an efficient and modern estate and builds on a number of rationalisation projects in 2018 including the move and subsequent closure of Vantage House and a building on Thorpe Road in Norwich.
'The next step would be to move people from Carrow House in 2020. By closing Carrow House it will save the council £400,000 per year in running costs and generate up to £1.1m by selling the building.
'Before we can move any more staff to County Hall, the north tower, lower ground and basement require essential renovations.
'These offices have had limited work done to them since they were constructed back in the 1960s. The essential renovation work is needed to improve building and safety standards and ensure the offices are fit for purpose.'
Councillors will be asked next week to agree to permit up to £9.3m more to be spent on the refurbishment, on top of £14.4m County Hall claims was earmarked in 2016.
But Steve Morphew, leader of the opposition Labour group, said: 'County Hall could finish up as a gilded palace with a resurfaced car park, yet without enough spaces for staff, at a time when people can only dream of their roads being resurfaced.'
Lib Dem leader Dan Roper said he would be writing to council leader Andrew Proctor for a 'full explanation'.
He said: 'This is a huge sum of money and the public will quite rightly want assurances that every pound spent is totally justified.'
The new bill for County Hall has emerged just before the council meets to agree its budget.
That budget, which could see children's centres closed and disabled people having to pay more towards their care, is being planned on the basis of a 2.99pc increase in the share of people's council tax which goes to the county council.
However, the refurbishment would be paid for from a pot of money for capital projects, so the cash for it could not have been used to stave off cuts.
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