I have a 37 year old drystone garden wall, about 30 cm thick. Would it be safe to raise a 2.4 section of it from its present 1.5 m height to about 2.3 m in order to provide a fire-resistant barrier to my neighbour's new domestic oil tank?
It would definitely not be safe to raise a 300 mmthick drystone wall which should be 750 mm in the base even with its present height of 1.5m to 2.4m. The wall needs demolishing and rebuilding to an adequate width ensuring the ground beneath is adequate to carry the weight. It's no use saying that the wall has stood for 37 years without failing when your neighbour sues you for a million pounds when it collapses and your insurance company won't pay out because it was structurally inadequate. Go to the Dry Stone Wall Association's web site and you should be able to abtain free advice either downloads or leaflets.