Polly its really too hard to give a % increase so many variables but to be brutally honest 10-15% is likely to be the range. But this is only the $ value added to the house and the land in a sale.....check out some of the detailed calculations being done in the USA over the last 30yrs in their urban areas...the value trees add to all the important elements of where and how we live is incredible.
Pacific Southwest Research Station Sacramento Tree Foundation: Greenprint Program: Overview
Cobra can and is used for fall arrest systems or branch support...tried to upload the cobra brochure but couldn't
The latest german cabling standard might be here on the site...vaguely remeber it being talked about either here or somewhere else (memory beer affected!!!)