In message
,
" writes

Possibly out of date, but I did check with our BCO when I did our loft
conversion, and he would have been happy with the two position lock
instead of the trickle vent...

Yes, it's still the case that a locked-partially-open position is an
acceptable alternative to trickle vents.

However building regs do insist on some of permanent background
ventilation.

That can be a whole-house ventilation system, ventilation bricks, or
window trickle vents.

There's a formula for the vent minimum area based, based on floor area
of the room, but can't recall the % right now.

Two years ago, I had a very small 'lean-to' extension built off the back
of the kitchen. It is only 2.3m x 2.3m internally, and is divided by a
wall+door into a toilet+shower, and a utility room. [Some would say a
'utility cupboard'!]

There are three windows (all two-position locking) and, after delivery,
the builder 'retro-fitted' trickle vent kits to all three. "New building
regulations" was his explanation.

There are also two extraction fans and vents. One is in the
toilet+shower part (which I expected), and another in the utility room
part (which I didn't - "building regulations" again).

Needless to say, the trickle vents have never been opened in anger!
--
Ian