In article ,
John Rumm wrote:

(I know this, because I speced trickel vents on the windows,
showed them to the BCO along with the two stage opening, and he
said that they would have been happy with the openers alone!)

Yes. Our two-stage dg windows were ok'd by the BCO, for
use with a woodburner in the lounge.

However, next door has a gas fire and they were advised
that non-closeable ventilation was a requirement.

--
Tony Williams.