What are the rules and requirements for adding a new soil pipe for the purpose of installing a cloakroom to a redundant boiler cupboard?

Hi you need to inform building control,and make planning aware of change of use of the room, then install a stub stack if your upstairs toilet is already vented with an exsisting soil stack , a stub stack is all you wold require ,exit though your walling and dig to nearest drain /manhole , and connect at benching level , put on test and then get building control to inspect and pass.

You would be best to talk to your local planning department as you may need permission to do so, and would probably need to be tested also. Its best to go through the stages as it keeps your paperwork right if you ever decide to sell the property on. Kind Regards Andy