I am looking to put a basin into my toilet. The toilet is situated in the
same room as the main waste pipe.

Next to the toilet is the bathroom which has a sink & bath. Both of these go
into a single waste pipe which runs down through the floor and then along
the ceiling below (boxed in). This runs directly under the toilet and where
I am looking to put the basin.

Is it best to try and tap into the existing waste pipe going from the
bathroom to waste stack for the new basin (will be very small so minimal
use) or should I just run a new waste pipe and box it in along the bottom of
the floor?

Rob Convery wrote:

Next to the toilet is the bathroom which has a sink & bath. Both of these go
into a single waste pipe which runs down through the floor and then along
the ceiling below (boxed in). This runs directly under the toilet and where
I am looking to put the basin.

Is it best to try and tap into the existing waste pipe going from the
bathroom to waste stack for the new basin (will be very small so minimal
use)

Sounds fine to me - certainly the easiest and neatest solution.

David