ProDave

When you say "water damage" are you referring to the brown stain on the shelf? That looks more like the waste fitting dripping, which should be easy to trace and fix. Agreed it could be neater, but unless any pipes are actually leaking I would adopt the "it ain't broke, don't fix it" approach. Some more pipe lagging would not go amiss

Ah yes I forgot that quirk of that type of valve. So you need probably a second person looking / listening to the boiler to see if it briefly tries to start as you push the manual lever on the 2 port valve as hard and as fast as you can to make it momentarily turn the switch on.

In your second picture, you can see the three 2 port motorised valves all connected to what I believe is the wiring centre above the 2 switches. First test. With everything OFF, go and turn on the UFH. Observe the 2 port valve for the UFH. Does that open? Second test, with everything OFF, manually opening the override lever on each valve should cause the boiler to fire up. Does it?

No mains cables should be at least 450mm deep IF it's steel wire armoured cable and properly terminated with proper glands you could get away with it but the point of burying it deep is so no future owner cuts through it when digging the garden. I strongly suspect that cable is just PVC or rubber non armoured if so rip it all out and start again with the correct cable.

See this diagonal support on the good side of the roof: You need to replicate that as near as you can on the side where the chimney used to be, then discard the inadequate prop. If you don't feel up to doing that yourself it would be a simple job for a joiner.

Leaving out the tiling of the hole created in the roof for now, I am more concerned at that pile of brickwork on the left, and that rather inadequate timber "prop" sitting on top of it. Can we see more detail of what that brickwork is and what state it has been left in? The actual roof tiling bit apart from the felt does not look too bad, what does it look like from outside?

While you have a substantial payment due, you hold a lot of cards. Get the negotiation done now about the snags. Once you only have a small payment due, then they have less incentive to dome back. List all the snags, like the oustanding plumbing work, kitchen doors and ANY other aspect of the job that is still pending or substandard. Ask for a site meeting to discuss this.

Contact them NOW and raise the kitchen door issue. Make it clear you are happy to pay the bulk of the invoice but you think this part is unfair. If they don't even know about the issue, then how do you expect them to deal with it?

I bet the instructions said use pea gravel only if not prone to high water table? Lets reset the conversation, apart from the occasional blockage between the final IC and the input to the TP, what is the actual "issue" you are trying to solve?

So you exposed the pipe between the final inspection chamber and the inlet of the TP and confirmed it was not kinked. Did you put a spirit level on that pipe when exposed? It must run downhill from the IC to the TP. We need to know if that is the case. If it does run downhill and the IC is not emptying completely, then the TP is filling too full. That could be because it is not level, or it could be an internal baffle has distorted. The baffles usually form a weir between the dirty and clean side of the TP. As for backfilling with pea gravel. That might be okay it might not. At a previous house the installer badly advised me and set a septic tank in pea gravel. The site has a high water table in winter, so if the tank was desludged (emptied) in winter there would be a very real chance the tank could float out. I dealt with the situation by only ever getting the tank desludged in the middle of summer. Even if it did not float out, it could possibly move if emptied at the wrong time.

There is a lot of confusion here and I suspect your lack of understanding is confusing the issues. So reading that you have mains drainage (i.e. you do NOT have any form of septic tank or treatment plant) and with the old house your foul and rainwater enter a combined sewer. Modern practice is where possible you do not put rainwater into the sewer in heavy rain it can overload the system. So modern practice is SUDS, Sustainable Urban Drainage. The idea is you try and deal with your rainwater locally within your property which might be a soakaway or might be some form of tank to fill up in heavy rain and drain more slowly into the sewer. Are you drawing and submitting these plans yourself? You really need an experienced drainage designer who knows the rules in your area to draw up a design that will work within the constraints of your plot and withing your local rules. Don't try designing this yourself.

This is what happens when the people that make the rules don't think it through. They want tenants to have security of tenure which translates to making it hard or impossible to evict a bad tenant. Landlords fearing getting stuck with a bad non paying tenant decide to sell up before the new rules come into force. Suddenly there is a diminishing supply of rented properties with increasing demand. Add in more regulation and cost, and requirements for minimum energy performance, and more landlords quit because they do not want the extra costs and in many cases can't afford to upgrade properties to meet the new EPC requirements. Don't blame the fed up landlords, blame the rule makers who don't think through their plans. Like some others on here, I would not advise you letting your property, I am glad to no longer be a landlord. With the cost of rent now, can "long term renters" not manage to buy their own property? (we sold our last rental to the tenant)

You need a more experienced electrician. The 3 metre rule is if you are relying on the supply head fuse, in which case your consumer unit must be within 3 metres of the electricity meter. For longer distances you need your own fuse protection and an isolating switch. commonly provided by a switch fuse. I use this sort, fitted with an 80A fuse so it discriminates from the suppliers 100A fuse. https://www.tlc-direct.co.uk/Products/FBFMS080.html

You will need to insure the building, and that will have to be a landlords policy not domestic. If you get an agent to collect the rent they will typically take 10% to 15% of the rent as commission. Statutory annual checks like smoke alarms, PAT testing, EICR and Gas safe. Ongoing repairs and maintenance. During void periods YOU will have to pay utility bills and council tax.

I think I know that design of house, a relative used to live in one. If so the good news is the walls of the "block house" are not load bearing. They knocked through from the kitchen into the blockhouse making that all into a larger kitchen. The downsides they lost the downstairs WC and the blockhouse had almost no insulation so it was cold.

Yes buy a caravan. Believe it or not they were designed for portable human accommodation. Either buy the largest touring caravan you can find and tow it to site yourself, or buy a static caravan which will need to be delivered.

I share your disdain with anything OR or BT related. Speaking as someone where BT would not even install FTTC where we are. We know Fibre passed the top of our road, and a cabinet there with a short run of copper to the houses would have given a pretty good speed. But they just would not and had no plan to do so. 4G / 5G is too weak and slow to offer a solution here either. Our saviour was a private wireless network installed by a private company now delivering us 100MB up and down, and if something goes wrong, a local office to phone where you nearly always speak to the same person. FTTP would be all well, but we built a new house 5 years ago and I really don't want to dig up the garden again and find a non intrusive way to get another cable into the house, even if it was ever offered to us. Their stubbornness means there are only a handful of people using OR supplied wired connections in the village now, almost overnight their entire network has been obsoleted. I really can't see them offering us any decent new wired fibre connection any time soon.

Left hand waste hose connector, the blue one. Is that plugged or open? Probably just wants cleaning with a bit of bleach?

Be careful "making to panel size" for some weird reason OSB is still available in imperial size, but don't build to that or you will curse when you try and plasterboard it as that is only available in metric.

Plastic boxes you just drill your own entry hole where required.

That video just describes how it works and how it is better in some ways and not so good in others compared to an UVC NO mention of reliability which seems to be the largely unspoken problem with them.

Fun university project. In the real world, I bet you could build a good stud wall quicker than that machine "prints" that wall, and the stud wall would be easier to work with, easier for services, and won't need skimming.

All we had for building warrant was a drawing showing where the stairs went, part of the usual construction drawings. We got ours from stairbox. I recall there was a tick box on their design page for English or Scottish regs as there are some differences e.g minimum width. They never specified the grade of wood they were using and building control never asked for it. All they were concerned with was meeting regs regards width rise and going and banisters meeting regs particularly must be vertical and a 100mm sphere must not pass through.

It is a long time now but we had to get the licence before BC would issue our building warrant. I don't remember it taking weeks, once SEPA had verbally agreed to allow is to discharge to the burn, I am sure it only took days.

Whilst I regularly encounter wind coming out of a socket box, it is usually when i have unscrewed the socket for some reason. To blow through like that even with the socket in place is unusual. It might be worth checking if they are all like this or just on one wall? That might pinpoint where the issue is. Is there any permanently operating extract fan anywhere?