Vent Pipes / AAV When Remodelling Downstairs Toilet Drainage and wastage systems and plumbing help, advice and answers

[edmolloy]

I am remodelling my downstairs w/c, which contains a basin and toilet. This is served by its own waste pipe, not connected to the main soil stack.

The basin waste is on 32mm pipework and connects to the 110mm waste pipe below the toilet waste. I understand that I need to include a valve to the waste system. Should this be in the 32mm pipework above the basin waste, or in the 110mm pipework?

See rough schematic sketch below.

Thanks in advance for any advice.