ᐅ Price difference between windows, fixed glass panels, and masonry?

We have planned a floor-to-ceiling double door on the ground floor layout (approximately 215 cm high and 150 cm wide, around 3.2 m² (34.4 sq ft) of glass area, two opening doors) and would like to replace this double door or double window with a fixed window to create a built-in window seat on the inside through a carpenter.

This means the position on the floor plan would remain the same, as well as the width and the top edge. The fixed glass element would then no longer be floor-to-ceiling at the bottom, but would end approximately 50 cm (20 inches) above the floor (then about 2.5 m² (27 sq ft)).

I expected a lower price because the window area would be about 20-25% smaller and the entire door mechanism would be eliminated. However, the construction company is now quoting an additional cost of 400 euros net / about 480 euros gross, because the extra 50 cm at the bottom requires additional masonry work, an exterior windowsill, and the fixed glass element is large and heavy in one piece.

The windowsill cost makes sense to me, though in the building specification we don’t have particularly expensive windowsills (exterior windowsills made of aluminum including anti-drumming foil with side closure profiles and drip edges). I did not expect about 0.8 m² (8.6 sq ft) of additional masonry to be a major cost factor, at least not more than the potential savings from the smaller window.

Does anyone have experience or advice on how to assess these costs?

Even if someone calculates this for you, it won’t help. You want it this way, and the contractor has given you their price. You either accept it or you don’t.
If you come back with, "But it will be cheaper, I want a lower price," they will probably say, "Fine, then the window will be installed as stated in the contract for the already agreed price."

The doors are installed by the window fitters taking out the sash, installing the frame, and then rehanging the sash. A floor-to-ceiling RC2 sash weighs around 100kg (220 pounds).
You can imagine that a glass element weighing about 200–300kg (440–660 pounds) requires significantly more effort, as handling something like that without equipment, relying only on manpower, is practically impossible.

Besides that, the contractor has planning work to do, drawings need to be adjusted, and the new element must be measured and ordered.
Do you think the people in the office handling all this work do it for free, out of goodwill, and kind words? If this takes an hour, you’ve already easily used up 100 euros (gross).

For us, the fixed elements were cheaper than the hinged doors. However, our window installer came with equipment anyway because we have a very large lift-and-slide door. Also, as the previous commenter mentioned, it’s quite pointless. If you’re lucky, you might be able to negotiate about a hundred dollars from the general contractor. Most likely, that won’t happen. You’d be better off spending your energy on other things.

I expressed myself poorly. The 400 euros net is the additional cost for about 0.8 m² (8.6 sq ft) of extra wall below the window. The fixed window itself is supposed to cost around 300 euros net more than the full-height casement door. This combination seems odd to me, so I’m trying to get a reasonable sense of the price range. The planning is being done by another company, so the construction firm has no effort involved. If I cause additional effort, I’m happy to pay for it (similar to NatureSys, where a large lift-and-slide door is planned just one meter (3.3 ft) away from the window in question, and the heavier component will definitely be needed). However, in this case, a total cost of almost 1,000 euros gross does not seem quite right.

Maybe. I also didn’t see why I should pay an extra 200 EUR for a thermostat from Vigour compared to a single-lever mixer. Or an additional 2,000 EUR for a bit of decorative pebble dash plaster that a plasterer can apply to the base in half a day, with material costs for the job maybe around 300 EUR.

The result? I didn’t take it...

You won’t really have another choice either. The builder has you on the hook and can basically quote you any price; you don’t have to buy it.

That’s always the problem when you want to make changes after signing. You have no leverage left... if necessary, the builder can always say, “Fine, then not. We’ll build it exactly as specified in the contract.”

That is absolutely correct and common practice; we have also declined several things before. I am only looking for an assessment of what is realistic, so I can better decide whether the extra speculative premium is worth it for me. 😉