Add:
"Increase tree cover by providing for one or more street trees to be planted to complement the site's setting, or nearby if no opportunity exists on public highway adjacent to the site."

Legally compliant/duty to co-operate - don't know.
Sound: see also representations on Policy NE8 Trees, Hedgerows and Woodlands, particularly our proposed wording to "Delete "major" in NE8, para. 6 and add "(off-site, ideally nearby, if no opportunity onsite)"." Extensions and alterations requiring planning permission can contribute to tree cover net gain through enhancing street tree cover, at least on public highways. (West Sussex County Council has a donate-a-tree scheme: https://www.westsussex.gov.uk/roads-and-travel/maintaining-roads-verges-and-pavements/verge-maintenance/donate-a-tree-scheme/ . Trees need to be watered appropriately during their first 3-5 years.)