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With this you'd be opening a whole can of wo... I'll get me coat!

Can't see either BC or EA going for that - no standards and no real data to back it up... makes you wonder if they ever got past the R&D stage ..?

Don't use foam .... Most refrigeration pipes use 10mm copper with a measly 9mm Armaflex or Insultube on them. Upgrade the insulation to a 28mm bore 25mm wall over layer of tuffcoat if you can get it as it's UV stable and use this to "seal" your hole through the wall, sleeving it through a 63mm down pipe. The insulation will compress enough for you to push it into the ducting and it lets you replace the outer layer if it degrades or gets damaged.

One of my friends did exactly this, worked well and no issue with BC.

A few tips if you are making the frame yourself. Try to design things to be simple to build. For instance a roof pitch of 45 degrees makes everything simple. If you had say chosen a roof pitch of 42 degrees, you would curse that decision at every single angle cut. Make the panels, each one pre covered in it's outer membrane (plus overlap) so when you put the kit together you just have to staple to overlaps and it's almost instantly watertight. Even if not in Scotland, I would suggest using the Scottish approach and clad the roof with solid sarking board (usually OSB or ply) then put the membrane on and almost instantly the roof is pretty watertight. Far better than the English method of flappy membrane or felt stretched across between rafters. The length of time it will take to erect the frame and get it pretty watertight is insignificant in terms of what exposure to water the frame can withstand. If you erect the first flor first and it will be a while before the second floor and roof goes on and you want a working platform, temporarily clad the floor in cheap OSB and regard it as disposable.

So the way I would approach that is to get a "workshop" built first that can accommodate a reasonable size panel with a flat slab base to work from. This could be as simple as building your link building temporarily. From this, get a 20ft container to store your wood - a double ended one is idea as you can load the timber one end and then have your chopsaw set up at the other - and then cut and feed from your storage into your panel assembly area. Completed panels can be stacked flat under a waterproof cover until you have enough to build your first structure. If you work on "standard" panel sizes using 8x4 models (think Segal buildings) then you can simplify your components accordingly and speed up your build. You may also want to build some templates or jigs to speed up the construction as its as simple as dropping the timber lengths in and gluing/nailing joints and panels.

My guess is it's a single phase supply. Most of the run done in large cable because of the length (they used wavecon 95 for the run down the road from our transformer) And then a junction to convert down probably to 35mm concentric for the run under the road and into the house. I saved about £1000 on my connection by digging a trench and digging the connection pit (so they didn't need to bring their own digger) which in my case halved the connection fee. I still think paying £1000 for them to pull a cable through a duct in a trench already prepared and just connect both ends was a lot of money. I then filled it all in after they were done.

I also could not achieve a meaningful saving by employing others to do it. Hence I'm putting the pot the cash I would have spent getting a mains connection towards going off-grid

I am planning to draw the air for my stove from the ventilated under floor space. It's on the drawings and building control have approved them so it must be okay.

I do wonder if they have quotes for a new 3 phase supply up to your boundary and then a single phase supply to you reading that. As stated by others, the contestable works are the trenching and road crossing, cable connections at both ends have to be done by the DNO. Are you going into mains drains ..? That's the other large cost for road openings a mole can't usually be used for that.

As long as you can show that the void has adequate unrestricted air flow then there should be no issues. The image half way down this page is take from the building regs http://www.stovesonline.co.uk/stove-ventilation.html

Looks like there are some nice gaps between the steels and the floor beams too so cables may not be an issue after all !

Plenty of online providers - look on the Planning Portal and they have a list of links for planning plans. Some you can even draw the lines on online and then print - cost is about £12 or so.