ProDave

The LA will almost certainly demand council tax from you.

Can you post details of this ecobricks you are planning to use? While I can see you want a practice piece for the main build, if only you were to make this cabin out of an insulated timber structure, you could make it comply with the "caravan" rules and hence not need building regs. Remember a "caravan" does not need to be on wheels to meet the legal definition of a caravan.

Logging the readings from all my meters * and logging them weekly on a spreadsheet is not a complete waste of time * normal suppliers import meter, separate metering of energy used by ASHP for space heating and DHW heating, solar PV generation and my own export meter (to measure how much has escaped the self usage measures and gone "astray")

This last week has been the first time my ASHP has fired up for nearly 2 months, to consume 9kWh of electricity to heat some hot water. For 6 weeks before that, the excess solar PV has done it all.

But if you have living spaces upstairs and bedrooms downstairs, you are likely to want upstairs warmer than downstairs. So you probably will want heating upstairs. The no heating at all upstairs model works when you can tolerate bedrooms sometimes a couple of degrees colder than the downstairs living spaces.

I have found this joists filled with rubble thing. I bet 100 years ago nobody calculated how the joists would react to this extra dead loading?

I marked mine myself, by measuring with a long tape measure, and then when I had it marked out I painted a single white line on the ground marking the centre of each trench. I then dug it lining up the centre tooth on the bucket with the line. My builder said it would never be accurate, then ate his words when he found no alterations needed to the trenches before pouring concrete.

Board one side, fit insulation, board the second side.

The halfway house I adopted, was I excavated and laid and compacted the sub base. I interpreted the 1M from the highway unless fenced literally. and banged a few sticks in and strung out a "fence" then I was working on my land adjacent to the highway but fenced off. I then got a highways approved contractor to lay the tarmac on my sub base.

Buy something like this. https://www.ebay.co.uk/itm/134888823698? I have 2 such systems in use, one Panasonic, One Sony. Both old and almost worthless as you can see from the pricing, but in their day expensive good kit. Still working well and sounding good. Though no doubt not as good as @Pocster £4K new system.

Pester him to do that drain pressure test. Only that will tell you if there is a leak in any part of the drainage system. Without that he is poking around at random, getting nowhere and not solving the problem.

That reads as someone that got the hump because you want to change something. Yes it will need some SE design. As for the rest pipes will be on display? How many pipes do you expect to have going into a vaulted ceiling and just where will they be going? Exactly NONE. They all go through the service void in the walls. MVHR vents are the only ones that can be tricky. But each case is individual and you will find a route. And just because the structure is built as a vaulted roof which makes building a good air tight well insulated building a lot easier to detail, you do not have to have all rooms open to the roof space. We had one bedroom fully vaulted with a mezanine floor as a feature. The master bedroom was partly vaulted with a higher ceiling but still giving a loft space above it, and the landing and bathrooms had normal 2400mm ceiling giving a larger loft space above them. So much flexibility and options you can choose as you go. And any loft space you do choose to create, will be just as warm and dry as any other part of the house.

Vaulted ceiling requires you have a ridge beam running end to end of the roof, and then probably larger rafters hung from that down to the eaves. It completely removes the need for the raised tie and all the truss members, leaving you a simple roof easy to detail. In our case we built as a hybrid roof, 195mm deep rafters, full filled with insulation and 100mm solid insulation above the rafters.

Why can't the slope go either way? surely whatever wall plate you support them on would be on a slope so the next joist along is a little lower etc. Is this not allowed? Are you forced to put them all level and add firrings? And if that is the case, whichever way you want the slope it would have to be dead flat in both directions then firrings added?

You don't need a site licence to self occupy a single caravan on your own site. The advantage of building as a caravan is that in some cases might be easier to get planning (or you might have it already) and it largely avoids building regs. There was a Grand Design building a "caravan" at 100 square metres and looking every bit like a house but it fitted the technical description of a caravan for which they already had planning permission. Why don't you build it as a house on proper foundations? What was your motivation for trying to make it as a caravan?

Post a picture of this narrow plot showing the adjacent buildings.

If you are building this as a "caravan" on a park home chassis, then a different set of rules apply to the dimensions of a "caravan" and those don't specify a maximum eaves height. They specify maximum overall dimensions and a maximum internal ceiling height of 3 metres and only single storey but no eaves or ridge height specifications. A "caravan" is also mostly exempt from building regulation. A "caravan" does not actually need to be on wheels, it must be transportable and lifting it with a crane onto a low loader qualifies for that so it need not be that high off the ground. For a self build you almost certainly will not be able to reclaim the VAT if building a caravan.

Span all the joists the shorter 5.2M dimension?

I built it entirely myself not from a kit. The only bit I bought was the rail and sliding gear I think intended for wardrobe doors. and the irnmongery. All the woodwork I just made.

The devil is in the detail. We used a sliding door in a previous house to comply with the (at the time) 2 doors between a kitchen and a WC. It was a sliding door between the kitchen and a short corridor. The door slid 100% into the pocket with a recessed finger pull on the end if you wanted to pull it out, and recessed handles each side to slide it. And it never ever got closed.

That's what I did. I fitted my door frames to match the doors, not the other way around, so no trimming of doors whatsoever and the ventilation gap left at the bottom. I also ommitted the door stop along the top of the door frame. It adds nothing to actually stopping the door, and adds another small space for ventilation.

Post some pictures of the work done and the gap. Who was the "Landscaper"?

I would "just do it" But I would not put it remote from the house as you have shown, I would put it adjacent to the house wall just around the corner on the side. Hidden by a tall fence and hedge I bet nobody will notice and you will get away with it. If you are served enforcement notice, then just move it around the corner so it is now on the back wall of the house and thus no closer to the highway than any part of the building.

the leak appears to be starting here What's going on there? Is that a hole in the tank? What for?

I have one of those. It is set up as a stupidly large tv in the smallest spare bedroom. But the real reason I keep it is as a spare. We have 2 large tv's in the two main rooms and if one breaks, I don't want to be rushed into getting a replacement, so I like to have a spare one that can be put into use while I either fix or replace the one that has died.