I am planning on similarly replacing an old poorly constructed “single glazed porch” in a year or so, when I have completed our current major building works.
Since the overall height and width of glazing is within your control:
Opening glazed units: I priced made to measure double glazed replacement windows locally in the Jura, and winced at the cost. Then I bought standard sized timber framed double glazed opening patio door and window units from Leroy Merlin, and had the new door and window openings in our old stone walls made to measure instead. It was a lot cheaper to pay the masons to adjust the openings to a standard frame size. These Leroy Merlin units in fact come from Poland. I found them flimsy and constructed down to a price, however once fixed in place I am ok with them, although it took a lot of adjustment and tweaking to get them to be lined up completely true. The Polish suppliers (GORAN - Wooden windows and doors) do better quality units, some aimed at the german market, and if ordering enough it might still be cheaper to get a quote directly from Poland and pay for transport. (I ended up doing this for 300mm thick insulating blocks, which cost about 1/3 of the quotes I had from local building materials suppliers in the Jura including the cost of the lorry from Poland. The blocks are superior quality to those available in France as well.)
Fixed double glazing: Often the cheapest form of sealed toughened glass unit to buy in the UK are the standard sized replacement double glazed patio door panels. I plan to investigate where best to get these at the best price in France, but would search Ebay and LeBonCoin. These can be set into timber for the areas of fixed glazing. However there is likely to be differential movement between timber and glass so careful detailing of the setting and use of thixotropic sealant with a flexible backing strip is a good idea to avoid any force being exerted by the timber on the glass.
Odd Shapes: The cheapest approach that I can think of for the triangular glazing on the two ends is to use polycarbonate sheet that can be cut to size by hand, drilled and screw fixed. (tradenames of Lexan or Makrolon [Bayer] have good anti-abrasion properties, UV and discoloration resistance). In the UK I would have gone to a supplier like Sheffield Plastics and bought a couple of 2m by 1.2m sheets of about 6mm thickness. Now its been used so much for Covid screening this should be readily available locally in France. Before Covid I ended up buying it from Germany via Ebay, and paying shipping costs, still cheaper than I could find from French suppliers. Since this would involve creating your own double-glazing, the screwfixing gives the advantage of being able to open it up again if you get condensation inside, but ensuring you have warm fully dry air filling it should be achievable at the right time of year. Careful desgn of the roof pitch can ensure little or no wastage. I do have concerns about the use of plastics from the sustainability point of view, but I have equal concerns about the use of concrete. I think one has to minimise both.
Timber framing: Walter Segal, the architect evangelist for self-build, advocated designing the building to make use of whole timber sizes without any cutting or waste. This kept the cost down, and speeded construction. It’s worth looking at how he detailed timber frames and in particular clamped joints to make it easy for self builders. Segal Method - Designing Buildings. THE_SEGAL_METHOD.pdf (ianwhite.info) I would also use galvanised bolts. However for joints that I wished to remain rigidly fixed I would add Galvanised Steel Double Sided Tooth Plate Timber Connectors (these are available in the UK under brand names such as Sabrefix and Telco, from suppliers such as Screwfix… unfortunately I have not sourced any in France to date. I bought some from Amazon.fr, and they were too flimsy to use, so the metal thickness is critical to resist flattening when clamped up!) For my “porch” I will instead be trying to reuse timber beams from an outbuilding roof that I plan to take down, so that will dictate some of the sizes.
fenêtre de toit: I have fitted a number of Velux roof windows in the past, and I think that their brand and their flashing kits are very good, and make the task easy. However you are paying a premium for the brand, and there are a lot of cheaper alternatives, even from Leroy Merlin.
Avoiding summer time overheating: The deeper the timber fins that you construct sticking out beyond the line of glazing and the more that you have an effective shading roof overhang, the less likely is overheating. This can be modelled in ‘free to use’ software like Sketchup. where you can put in the latitude and longitude and study sunpath and shading through the seasons. Planting deciduous trees or considering projecting timber supports for deciduous creepers or vines can also be sensible to consider at this stage. Staff at Sussex University did an experimental study of the cooling effects of growing deciduous creeper over openable windows, and found it cooled significantly. There is a similar supposedly innovative study from TU Delft An experimental study of vertical greenery systems for window shading for energy saving in summer | TU Delft Repositories
