Renfrewshire Council could have to find as much as £45 million for a second primary school in Dargavel Village after botched calculations left the initial building facing capacity concerns.
The eye-watering figure has been released as part of a report confirming education chiefs are now planning for a facility with space for 800 pupils at the housing development – not around 700 as was indicated just months ago.
Talks between the local authority and BAE Systems, the landowner, are still ongoing in a bid to identify a specific location for the newbuild.
It has also been revealed an extension is needed for Park Mains High School, the associated secondary school of Dargavel Primary, to increase its capacity from around 1,600 to 2,000 – at a cost of between £27m and £30m.
The council expects a proportion to be met by BAE through developer contributions, but it is likely remaining fees will be met through prudential borrowing.
Council leader Iain Nicolson said: "I fully understand the concerns of parents and carers in the local community and would like to acknowledge the collective efforts in progressing planning to this stage.
"Council officers have been working hard with the parent council to develop plans to accommodate the projected number of pupils in the immediate and longer term.
"Strong progress has been made towards the immediate actions and detailed planning, guided by the revised projections.
"We remain fully focused on getting the right solution and ensuring we continue to deliver a high-quality learning and teaching environment for all pupils.
"Continued engagement with all parent councils is critical to this work and we will always seek to work in partnership, listening to the views of everyone impacted."
The saga emerged in November when it became clear the primary school in Arrochar Drive, Bishopton, wasn't big enough to meet the needs of its growing catchment area.
Initial calculations, dating back to around 2017, projected the roll wouldn't exceed 430 pupils.
However, analysis from consultant Edge Analytics recently predicted the number of pupils could peak at 1,131 by 2033 – but also said space may be needed for as many as 1,500 if demand was to continue long-term.
Council bosses have apologised for the blunder, pledging to fix it, and, as a result, six modular classrooms – with room for up to 198 pupils – will be introduced at the site, which currently has a maximum of approximately 530 if necessary, over the summer.
Despite this measure, it has been acknowledged demand is "likely to exceed" the increased capacity of the existing school as early as August 2024.
This would create a requirement for the council to apply its admissions policy from that point, with the nearest alternative being Bishopton Primary School.
Members of the education and children's services policy board will be asked to agree the revised figure of 800 for the new Dargavel school and give planning for an extension at Park Mains the green light on Thursday.
Meanwhile, the outcome of an inquiry into the roll error, which is being led by a former local government chief executive in David Bowles, is expected in June.
Councillor Nicolson added: "It is important we get the right solution in place to continue to provide a high-quality education for young people in the area and engage with the parent councils and wider school community as these plans progress.
"An independent external review is investigating the circumstances.
"It’s right we provide an outline of the current estimated building costs now to help everyone understand the scale of the challenge ahead while detailed costs are developed as design work is finalised and these will be brought forward for full consideration."
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