Which trade to ask and budget?

Hello, my toilet waste pipe outside does not vent. It’s a closed system and gasses are coming into the house. We have a loft extension and the waste pipe needs extending from outlet from loft bathroom to above roof line. Work should be possible with ladder and from flat roof. Waste pipe on wall in side alleyway between houses. Would be grateful to know which category this falls into and have no idea of cost. Thanks for help, all best Simon