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I also used VP400. It survived being left uncovered for months.

Only issue I can think of is that side windows may now have to be frosted glass to be permitted development (or is that only first floor side windows?).

Take a look at this page.. https://www.ledspace.co.uk/pages/led-strip-kit-builders Their "kit builder" works out what you need to make complete mains powered light(s) including the power supply. It works out which power supply you need based on the length of the light fitting and/or number of fittings. I don't think the power supply fits in the channel if that matters. I'm sure someone must make a similar ready assembled unit if you don't want to build one.

There are also things like this.. https://www.ledspace.co.uk/products/flat-corner-led-profile?currency=GBP&utm_medium=cpc&utm_source=google&utm_campaign=Google Shopping&campaignid=14654411474&adgroupid=128524150738&keyword=&device=t&gclid=CjwKCAjwsJ6TBhAIEiwAfl4TWPewxg9PtEarXrFsksh6yetRWdh3Sep9GRwV_yek5PZFSSbY3XdcMxoCRaIQAvD_BwE

They look perfect if the heads fit the brackets. I would buy a few spares and drill holes in some scrap wood of same type to work out what pilot hole to use on the real thing. Don't want them to be so tight you shear off a head when in the wall.

This describes the ground beam approach.. https://www.diyfixit.co.uk/building/bridging-a-drain-run/

That's exactly what I meant in my post above. You may only need one home made beam in the middle wall and that will be shorter. You wouldn't use a curve at the front. Probably a 45 degree section into another inspection chamber on the original route.

We have beam and block with insulation and screed/UFH on both ground and first floors. Wish we had fitted more insulation on top but we don't notice any hollowness to the ground floor.

Get someone to sew up some fabric tubes?

I wouldn't do that. The pressure treatment is typically only a thin layer on the outside. I'm in the process of losing a fence because I used a strimmer around the posts. It damaged the pressure treatment layer and allowed the posts to rot. Other posts not strimmed are fine.

Oops @prodave posted while I was editing mine above.

or. Someone probably makes big stainless steel wood screws. These have hex head.. M8 X 120mm https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.ebay.co.uk/itm/190819704599&ved=2ahUKEwjf9q6Xs7H3AhXloFwKHRuHA10QFnoECBUQAQ&usg=AOvVaw3grdgHzL6PqOykR-vAEoDP

Think you want bolts that have a wood thread one end and a nut thread the other. You lock two nuts together to act as a head to screw it I to the wood then remove the nuts, fit bracket and use nuts to secure it. Cut off excess. Should be stainless steel. See if others have better idea.

https://forum.buildhub.org.uk/topic/18177-steel-beams-fire-encasement/ I believe the conclusion for a 2 storey house is it needs 30 min protection to beams.

As I see it the situation is the same a glass balcony so not surprised. I would ask window suppliers to quote for the windows and point out the first floor windows must be designed to meet BS6180 due to their height above the floor and ground levels. See what they say. Edit: There will also be rules on which bits can open. Don't think anything below 800mm or is it 850mm can be opening unless there is a restriction?

The only think I can suggest is moving the pipe further under the extension so you don't need the longest of the ground beams. If the pipe passes under walls at 90 degrees that minimises the length of the beams needed. If the pipe could be moved under windows or doors it might be possible to use concrete lintels to bridge over it as there won't be so much weight above. The weight will be carried by the lintels above the window/door as normal. Would need to discuss wit the SE.

I would tell him its not feasible because the only route is over land you don't have control over/permission to cross. I would draw an enlarged site plan showing the neighbours houses with distances marked. I would also provide a scale cross section showing how the ground level slopes. Show the nearest wall of their house, the fence and your proposed extension. Mark it up in big letters "Nearest Neighour" and "Proposed Extension" so the planners can't get them the wrong way around and think you will overlook them. I'm afraid that asking for more information at the last min is also their way of delaying processing the application. They can now argue you haven't yet provided all the info they need.

For comparison.. Spacetherm Aerogel - Lambda value: 0.014 W/mK Typical PIR - Lambda value 0.022 to 0.026 New Kooltherm K100 - Lambda value: 0.018 W/mk So yes Aerogel is better but you still need a decent thickness of it. It's not quite twice as good as standard PIR. .

Yes. In theory you don't need a ventilated void if its a breathable membrane. But the void is also to allow the membrane to drape. If you push it against the underside of the batten dirt can accumulate and form a mud that stops water running down. A pool of water can form causing the batten to rot.

What is the white pipe on the right hand side? Is it just leaning up there or in use? Perhaps replace that with a stack and connect the new into that?

If its a cold roof (insulation between/below rafters) the top edge should be ventilated. Is that why they left gaps under the lead? Not the best/correct solution. Google Abutment ventilation or roof ti wall ventilation. Example ventilation system.. https://www.wienerberger.co.uk/products/roof/systems-and-accessories/top-abutment-ventilation.html

Spray foam can be bad news.. Portsmouth News: Portsmouth woman's heartbreak as foam insulation scam makes her home virtually worthless. https://www.portsmouth.co.uk/news/people/streetwise-portsmouth-womans-heartbreak-as-foam-insulation-scam-leaves-her-home-virtually-worthless-3658803

Check the purchase price includes the FIT payments. I have heard it is sometimes possible to "take them with you" when you sell up.

My neighbour had some fantastic hinged iron gates on 8ft brick pillar constructed last year. They lasted a month before he had to get the builders back to fix them. That involved taking down the brick pillars and rebuilding them stronger. important to get it design right. Are your gates going to be sliding or hinged? Found.. https://community.screwfix.com/threads/iron-gate-footings.211575/