Lean to back of house

Hi personal I would have the lean to below the flu as the gases coming from the flu need to be free to flow away.

Hi, not sure if you can see my profile, get a 90 degree corner piece, and a up pipe and go though the roof of lean to, like a chimney in a way, there are plenty on the market. Aaron

Gas condensing boiler flue is twin wall the middle pipe takes out fumes and the external pipe draws in air I do not see why the existing boiler flu cannot be extended up through the extension roof it can be hidden with a cupboard or boxed in making sure there are inspections hatches at any joins as these have to be checked annually .also the flue will have to be extended by a gas registered plumber ask him for a second opinion maybe the option of taking a new flu straight up from your boiler the plumber will know best but the flue is not a major issue and best to check with building control also as you are changing an important fixture to the heating system hope this makes things clearer