Had a manhole in my last property conservatory ( terrace property ) .
Before extending had to remove the access and install drainage thru to interceptor and install new access outside my build line to satisfy water company ( Thames water ).
Good look with that , our office space had sash windows to one side ( listed building) all 60 were hardwood , modern double glazed with built in draught excluders and smoke seals .
You could still post a letter through the gaps .
Better to install secondary glazing.