is the idea behind a  DG unit in a garage one of Aesthetics  (to match remaining house units!!), or to help insulate the garage!!

if it is for aesthetics only, then there is no problems, it just like installing a single skin, in that form a window is a window!!

however if for insulation values then, yes it is still ok to do so!, but, it will have little effect thermally due to single skin BkWk , and all the gaps around  the garage doors!!

just one other item, why if you are just replacing a window, like for like size i take it, would you need to contact the BCO... ???

Wolf, though the BCO needed to have his say for the replacement of windows if not being installed by a FENSA member.

An off the shelf DG window would be cheaper than getting a special with single glazing.

Jason

Now there was me thinking that FENSA etc, is all about the actual spec for the windows them self!! not the actual fitting!!  a D/G window has to be at a certain spec level to be allowed to be fitted these days to comply with Part L,

Well thanks for that tid bit of info  Cheltonian, and welcome to our humble forum!!!
the reason i posted that statement was becausee the Heritage trusts i manufacture the odd window for; gave ME a few pages of spec from Fensa , so that any timber framed window with D/G units in comply to the regs they've highlighted for me!! strange, me thinks i will have to ask them about this!!!

Yep!! i know the graded buildings don't , but for about the last 9-10 months the heritage people have been doing a revamp on some of their offices, and don't like using Plastic/metal windows, so keep opting for timber replacements with D/G units in them!!! Bloody pain they are too! all silly sized panes, fancy catches(usually the cast iron ones!-in BLACK!!) and of course i have to keep doing the windows, as i like the challenge of the Newels, spindles and other ancient timber items i have to duplicate for their listed/grades stock!!!!  "swings and roundabouts" i suppose...Ho hum!!!

CW

Ultimately that's what I plan to do, but I'm looking into ways to make it demountable, so that if the BC turn up I can change it back without too much trouble. I kinda work in the trade ( Civil Eng ) so have access to shed loads of 'gash' timber, marine ply, electrics and stuff, all of which can help to keep the cost down
My only concern, however, is that should anything ever go wrong in there - fire, flood DEATH -  then I'm sure the house insurance would be invalid.

Gammy