Is a plumber right for this job?

[Springmachine]

I'm about to creat a downstairs toilet.

I have someone to build the wall, the plasterer and the supply of the sanitary ware.

I know il need a plumber to run the hot and cold water pipes to the sink and toilet but would it also be a plumber who connects the toilet waste to the external drain?

It would need to run under the floorboards for 3.5mtrs and then connected to drain underneath block paving.

Is there any other trades I would need to do so?

Well my dp is a carpenter and he could do all of those things for you. I don't believe you need a plumber to run the pipes. I do think you need building control type inspection if you are connecting to the main drain though.

In short you need a builder, someone competent enough to cover all the tasks needed without calling upon other trades.

You may need en electrician if you are having a showe installed.

A general builder would usually do the whole job in my experience, but if you have already got people doing the other bits you mentioned then yes you will probably need a plumber too.