Just before the pandemic, we paid for our windows to be measured for timber windows.
Our financial situation being different now, we are opting for Upvc double glazing, which we’re approaching by replacing a couple of windows at a time.
Having tried myself, I know the two windows being replaced are very awkward to measure the outside brick void from the inside.
The original measurements given for the windows were 1300 x 1060 our measurements were similar at 1300 x 1070
This recent window fitter measured 1265 x 1080 .
The writing should have been on the wall really , when he measured the windows; briefly putting his tape measure outside - it was bowing as he took the measurements.
My understanding is there should be a 5mm - 10mm expansion gap all the way around the window.
These latest measurements versus the previous two are 35mm smaller on the height and 10mm bigger on the width.
Correct me if I’m wrong , but having larger gaps around windows will potentially lose thermal performance.
I am wondering how one would effectively insulate around a window with oversized expansion gaps.
Another problem I am experiencing is finding a fitter willing to use the insulating expansion tapes .
Any advice , shared experience, fitting or fitter recommendations would be greatly appreciated here .
Thanks in advance
Carla
Is there a carpenter that you trust? If so buy on supply only and get your carpenter to fit them. My experience of carpenters is that they are very meticulous and open to newer ideas, such as insulating expanding tape perhaps. An established window fitter “knows everything” about fitting windows and isn’t interested in complicating matters with new fangled ideas.
The size of expansion gap is proportional to window size, as expansion itself is too. Unfortunately I can’t offer figures.
Edit: I’ve found the figures!
Coefficient of thermal expansion of uPVC is 0.06-0.08 mm/degree/metre so a typical 800mm wide [window or] door between winter (say 0°C) and high summer (say 40°C) = 0.07x0.8x40 = 2.24 mm.
Expansion doesn’t seem to be a major issue. It’s probably more of a matter of “wiggle room” while fitting.
Edit 2:
Modern windows are supplied with right angled corners.
Old openings often aren’t truly squared.
Therefore even though the measurements say a particular size it doesn’t mean that a replacement window that size would fit in the gap.
e.g. What size square fits into your rhombus? Or rectangle into your parallelogram?
I think that’s an excellent idea.
Thankyou
I’ll keep us posted. 
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A 10mm gap around the window should be enough wiggle room (measurement minus 20mm). You could always make a dummy frame with batten to check the measurements.
Have you looked at Klar? Buy Windows and Doors → Up to 45% web discount | Klar I wish I found them at the beginning of my project. I just managed to get 2 doors and 1 large window aluclad timber frame from them. The rest are all aluminium frames with either double or triple glazed. Wherever you get them, ask for a triple glazed price for comparison, I found it wasn’t actually that much more expensive than double.
Also whoever fits your windows try to get them to watch this 1st https://www.youtube.com/watch?v=2d_g6NQjYXc&ab_channel=tremcoillbruckGroupGmbH%2FtremcoillbruckGmbH%26Co.KG
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