Council managers have admitted a series of errors after work to extend a park and ride car park cost £116,000 more than expected.

The estimated price of extending the Pear Tree park and ride car park, Oxford, was initially put at £292,000 in July 1995 and set at £431,000 four months later.

But delays in starting the work, changes to the contract, and use of extra stone, meant the final bill came to £547,000.

Director of property and leisure at Oxford City Council John Arnold said none of the extra costs were "ultimately avoidable" but admitted there had been a string of errors, including:

Failing to report what had happened

Incurring additional expenditure without necessary authority

Errors in preparing the contract. Alex Hollingsworth, chairman of the city council's highways and traffic committee, said action had been taken to ensure similar errors did not happen again. He stressed that extra stone used to resurface the car park now meant it would last twice as long as the ten to 15 years originally expected.

Cllr Hollingsworth said: "We admit errors and have an assurance they will not happen again.

"While one should criticise the extra spending, the car park will last twice as long."

The council initially planned to spend £292,000 on an extension to the car park. But problems securing a lease meant councillors agreed to resurfacing the whole car park - to increase capacity by 240 - at an estimated cost of £431,000.

The work was due to begin in summer 1995, but delays meant it did not start until January 1996 - by which time the car park's base was too wet and had to be strengthened by more stone. The amount of stone needed for the project had also been underestimated in a survey.

Additional stone cost £93,000, other delays and extra costs amounted to £23,270.

The money is expected to be found from developers contributions and a county council loan.

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