ProDave

Bypass mode, as it's name suggests, bypasses the heat exchanger. In a hot summer, that will result in HOT outside air entering via the mvhr with no cooling effect. Leave it in normal mode. Then outside air that is hotter than inside, will be cooled by the cooler extracted air. I have struggled to find any time when it is actually advantageous to use bypass mode.

I would not use LHZS cable, I find it horrible stuff to strip and use. Standard PVC for me.

Why do you need a grease trap? It seems you just need more robust detail on the drain you have to stop soil entering and blocking it?

I await your explanation of how it will cost upwards of £300K to install your heat pump........

My self build journey, to illustrate how things have become harder. First self build 2003. Moving from England to the Highlands. A big motivation of self build Vs just buy a house, was we could buy a building plot and a static caravan for cash without having to sell our old house. If we had gone for the "simple" option of just buy a house we would have been trying to sell and buy at the same time under 2 different house buying legal systems which just sounded like a nightmare. Plus a self lbuild had been on my bucket list for a long time. That first house the only technical design input was from the timber frame company. Total cost of all drawings planning and building warrant £2K No surveys. TF company was paid to build a and watertight shell and we fitted out the inside employing trades as needed. House prices here were rising fast then, and by the time we had finished house value comfortably exceeded build cost. That all worked out well. IF ONLY it was still that simple. A few years later we decided we wanted a smaller house, (done the 5 bedroom thing) and were lucky after some searching to find a plot 2 doors up the road from the first one. The plan had always been start the new build, put the old house on the market and sell it to finish the new build. Our timing was poor. This was post 2008 crash and the market here was dead. The old house was on the market for 2 years and had just 6 viewings. My work as electrician took me all over the place, and I was seeing the same houses everywhere just stuck with a for sale board that had been there well over a year. All the agents and any "helpful" adviser could say if it's not selling, it is over priced, reduce the asking price. That would be all very well if you could reduce your asking price and then reduce the asking price of the house you bought. But self build does not work like that. Try telling a builders merchant house prices have crashed so you want all building materials reduced by a similar amount and ditto tradesmans wages. If we had reduced the price to distress sale levels, we could very well have ended up selling our 5 bedroom house for less than the build cost of the replacement 3 bed house. There was no way I was paying to downsize. Our build ground to a halt with just me doing what I could but held up by bigger things needing doing. Our saviour from this situation came in the form of a lady that wanted to buy the house and run it as a guest house, but could not get the funds to buy it without accounts, so her offer was to rent the house for a few years and then buy it. So we accepted and reluctantly went back to being landlords. That did eventually work out. It ended up being 7 years rental before she finally bought it, and those 7 years of rent, pretty much paid for the completion of the new house. So it worked out financially better just because of that. The actual sale price of the 5 bed house was only £50K more than the build cost of the new 3 bed house. As others have said a self build now likely costs more than the market value unless some house price inflation saves you. As for complications and red tape. The old pay the TF supplier not a lot for all the drawings seemed no longer available. We ended up paying an architecturtal technician and a SE to produce all the drawings that cost us £10K. We didn't have to do any surveys or that nonsense. The only other bits of paper we had to pay for (but not on the first build) were air test, and as built EPC. Building control seemed more stringent on the second build, with the added irritation that we had to keep paying an extra £100 per year to keep renewing the building warrant up to completion. And self build site insurance was a LOT more expensive second time around.

So go back to SE2 and say you don't want wind posts, ask for her alternative?

So what drawings are you working to? What did the original SE specify to give adequate racking strength? Why has the new SE changed that? Surely he will have provided proper drawings?

Pictures? There is an alternative, I think called an over fascia vent. This has the front vent holes facing downwards so less likely to be troubled by a membrane hanging down the front.

My designer originally proposed them. I took one look at the detail, which added complications to the founds for the wind posts to bolt onto. And then appeared to compromise the insulation by having a steel goal post built into several of the timber frame sections. I asked for a re think and he came back instead with a standard timber frame, but modified to have two layers of OSB with staggered joints to give the required racking strength instead (normally only 1 layer used)

I would take the skirting round the corner with a nice corner joint. You may need to trim the top of the skirt to fit under the unit and would only work if your skirt was no thicker than the end panel of the unit.

Personally I would not combine the utility and bathroom run, instead I would have a separate run straight out from the utility room and an extra IC along the top as shown in this plan.

I don't believe CIL applies anywhere in Scotland. If it did, it would have been written into your planning permission and you should have got the exemption before you put a spade in the ground. I

I recon from experience 2 days of electricians time and 2 days of plumbers time to install one on a new build. And I am in the camp that does the sums on that plus materials cost and wonder just how most of the companies that quote get figures as high as they do. If you are going for self install, then look at the other option, some companies will supply the kit for £0 using the grant for you to install. MCS sign off for a heat pump is not mandatory, in fact I struggle to see what that expensive bit of paper will actually do for you.

What you can do for that circuit is separate over current protection and earth leakage protection. That would tell you why it is tripping (though not stop it) which would make solving the issue easier. And a separate circuit for the freezer would also protect it from something else in the garage tripping it.

There have been a few developments around here, all small scale, where a number of plots were sold individually for self build. Yes it can take a while before they are all finished, but generally the results are good. If this link works https://maps.app.goo.gl/S3TDGbpQvWBsd7a59 All the houses to the west of that point were sold as individual plots, I wired one of those as a new build, and another when they extended into the loft. These are the detached houses around a green. The houses to the east of that point were all built by a developer. Guess which end of the street I would prefer to live.