Replacing old UPVC window with a smaller one... how to fill gap?

[STORYTELLER]

So,

as it says... I will replace upvc window with a smaller one... off-the shelf. It could be about 3-7 cm narrower, maybe upto 17cm shorter - leaving new window higher up.

Question is how to fill the gap? Is there a reg.? Can I used a piece of timber that suits the gap? if window is a lot narrower can i put in a framework with noggins? then overboard inside and outside with cement fibre board, cover outside to match the pebble (ish) dash render?

Or would I have to use brick/block/aerated concrete, cut thinly and mortared in....? or?

can seem to get a straight answer online... maybe there isn't one?

why DIY? quoted £1000 for made to measure window with fitting! vs £280 for off shelf unit.

appreciate any pointers.

many thanks

 

[freddiemercurystwin]

£1000 sounds extreme, have you tried a few other suppliers? You would use frame extenders to fill the gap, normally available from the same place you buy the window so as to ensure compatibility.