A Bristol school that is bursting at the seams could have to wait for new buildings because the cost of a much-needed revamp has more than doubled.

Westbury-on-Trym Church of England Primary school has already begun taking in extra pupils in advance of a £2.7 million refurbishment and expansion scheme.

But now a report to city councillors reveals that the necessary improvements to the 40-year-old buildings would actually cost £5.7 million – not much less than constructing a complete new school for £6.9 million.

The increase is said to be due to the extra work needed to meet disability discrimination requirements and new building regulations.

Officers say an extra £3 million or £4.2 million cannot be found without taking it away from planned primary school rebuilding schemes in other parts of Bristol.

They suggest that a possible solution would be to provide temporary classrooms at Westbury- on-Trym to meet the increase in pupil numbers. It could then consider the major works in four years' time or later.

But the school's governors, who began talking to the council about refurbishment four years ago, are not happy with that.

The school, built in 1969 for a maximum of 315, already has more than 370 pupils and is set to increase to 420.

It has 60 children in its reception classes this year and expects to have the same intake in September.

Chairman of governors Andre Hart said: "It is very frustrating. We don't want temporary classrooms, although we accept we will need them this September in the short term. Too many outside classrooms take away playground space. What we need is a school with 14 classrooms fit for the 21st century and the facilities to go with them.

"We understand that the council has limited funds and we don't want to take money away from other schools. But our building is not fit for purpose. It is expensive to heat, has poor ventilation, and is not eco-friendly."

The school has no secure reception area, its hall is too small, computer facilities are inadequate and the head's office is in a cupboard.

It already has two 20-year-old temporary classrooms which have to remain in service even though they are well past their use-by date.

Head teacher Pam Derham said she had begun taking in extra pupils to help the council by easing the high demand for places in north west Bristol. This had been done in the expectation that work would begin on the revamp last summer.

She said: "The extra numbers are putting pressure on the whole building.

"Our parents, staff and governors have been very patient – but our children deserve better than this."

She said there was a concern that the school was not being given priority for funding because its pupils were achieving well in spite of the poor facilities.

Westbury-on-Trym was rated outstanding by the learning watchdog Ofsted in 2007.

The funding shortfall was revealed in a report to yesterday's children's services scrutiny commission by service director Kate Campion.

Updating councillors on the first phase of the council's citywide Primary Review, Ms Campion said cabinet would have to decide whether to delay the work at Westbury-on-Trym further, or to make the decision to push forward with a partial refurbishment.