 OPPONENTS of the controversial Beauly-Denny powerline have called for alternatives to be reconsidered after it emerged the project's cost has almost doubled.  The overall bill for the 137-mile line to take renewable energy from the Highlands to the central belt has risen from GBP323 million to nearer GBP600m in seven years.  The project, which aims to replace the existing 132kV line with a 400kV overhead line, was approved by energy minister Jim Mather last year despite widespread opposition from environmental campaigners. the decision followed a lengthy public inquiry in 2007.  Last night the Scottish Government insisted the powerline remained vital to its flagship renewables drive.  But Helen McDade, head of policy for the John Muir Trust, said: "The economics alone make it vital to reconsider, but we shouldn't lose sight of the fact that this line will lead to wanton destruction of the Highland landscape.  "Given that the energy regulator Ofgem is increasingly recognising that sensitive landscapes need to be given more consideration when planning transmission routes, it would be ironic if Scotland remained stuck with this white elephant."  a Scottish Government spokeswoman said that investment in infrastructure was vital to achieving its target of generating 100 per cent of Scotland's electricity needs from renewables by 2020.  "Beauly-Denny is essential to harness the vast renewable potential in the north of Scotland. The initial costs of the Beauly-Denny project refer to 2004 figures and have been subject to revision over time and we understand that estimated costs have increased due to the detailed engineering studies.  "The costs of transmission represent 4 per cent of an average domestic bill and it is for the regulator to determine what costs can be passed to consumers.  "The long-term enhancements to the grid are a necessary part of the development of a secure, affordable and sustainable fuel mix and a low-carbon future."  Scottish &amp; Southern Energy's (SSE's) estimate for its section of the route was GBP245.2m in 2004. It has now told Ofgem the construction costs have risen to GBP473.3m, although the regulator has restricted the investment to GBP458m.  ScottishPower, which is responsible for a 12-mile section of the line in Stirlingshire, says its costs have risen from GBP77.7m to at least GBP100m.  The rising costs have been attributed to the delay in getting to the construction stage, the increased cost of the raw materials needed, particularly steel, and measures imposed by the government to mitigate against environmental damage which were not factored in to the original estimates.  Ofgem, however, says that the cost rises affecting the project will have little impact on consumers who will have to pay just 10p a year extra for the next 20 years as transmission costs make up only a very small part of a domestic bill.  SSE says preliminary work is already under way on the project, with full construction on the replacement line, including the erection of new pylons, ready to begin later this year. It is due for completion in 2014.  a SSE spokeswoman said: "The original estimate did not include a rise in the cost of raw materials, improvements in safety and construction standards and additional costs of rationalis 