how close can I build next to a BT Telecoms cable?
What do you mean , on your own land , or in a public area ?, an overhead Openreach ( not BT ) cable or an underground one ?
An underground cable. And if you would know for both a public space and private land that would be great, thanks.
You would need to ask Openreach about this, as its nothing to do with BT Retail, who run this forum.
https://www.openreach.com/help-and-support
The depth of cover and separation between other services is shown on the Openreach site ( for developers on ‘new sites’ and utilities excavating near existing plant ) , but it’s not something a regular consumer would need to know , however if you suspect some buried lineplant is in your way of whatever you are intending to do , you don’t build on top of it , if its deep enough in the ground, it’s diversionary works ,and should be moved out if the way , that generally the requester pays for but is for major works , like widening a road , but posting in here , presumably it’s a much smaller thing you are intending to do .