Having knocked off most of the render around my house (we are leaving those walls that will not benefit from a new lime render) the original reveals for the windows and doors are fully exposed. This then gives one a chance to see how big the original frames would have been.
One downstairs window has a gap of 3 inches / 75mm each side, so over time my window has effectively been shrunk by 6 inches / 150mm. That’s quite a lot of light that I have lost!
What happens is that when windows are replaced we don’t hack off the render or plaster to see where the structure is. The windows are measured using the visible reveal. No installer wants to estimate depths that they cannot see and most people don’t want to have their reveals knocked around just to get an accurate measurement. So windows get smaller.
When the windows are installed, they do have to ‘mess’ with the reveal and then you find out that you could have had a larger window, but by then it is too late. The new windows are fitted and the reveals are extended so that the window frame is enclosed with plaster etc. Next time, of course, your reveal is now smaller and the process repeats itself.
I would estimate that around 0.5 – 0.75 inches (12 – 18mm) is lost per fitting, so this would mean that this window has been replaced about 5 times since construction.
One of the other common occurrences with this process is that this gap is often filled with that days newspaper. So, you can often catch up on the news from a couple of decades ago! Nowadays fitters tend to use expanding foam to seal up these gaping holes around your windows which is not so much fun. I am not a fan of this foam and so I have used some fluffy insulation in the gaps to provide a little more warmth (and more importantly) fewer thermal bridges around the edge of the window.
So, if you want to keep your windows as big as possible, you will need to allow fitters to remove the plaster around the window so that they can accurately measure the size of the original window void. This is unsightly and dusty, but it is the only way that a company can reliably measure up. They are not keen on making windows too large or tight as it means that they are responsible for getting it right, so their eye on caution is understandable given the costs of re-making the frames and glazing.