BC have signed off.   We have 5 air vents on either side of large extension but due to the walls being SIPs then periscope vents could not be used (no cavity for them to rise in). We used straight through vents (in agreement with architect and building control).   10 vents have 150mm clearance but 2 of them - due to proximity to neighbours foundation and wall (600mm away) are at ground level. I might be able to dig down a little but not sure I can get 150mm clearance that war