ProDave

For us the decision was made that we WILL self build before looking for the plot. MOST of the potential issues you list can be largely solved by you being actively involved at least as project manager if not actually doing much of the work, and making sure the detail is done right with no corners being cut. The thing that went "wrong" for us was assuming we could just sell our old house (we needed some of the money from it to build the new one) Never under estimate the fickle housing market not long after a "bust" where the market was stagnant and very little was selling. We gave up after 3 years on the market and agreed to let it instead. That left a £70K hole in the budget to build. From that we became masters of building as cheap as possible (cheap price not quality) mostly by doing so much of the work ourselves, and we have ran for 5 years as a "build as you earn" funded largely by the rental income from the old house. On the plus side, although it has taken longer than we ever imagined, we have built the new house now without touching the capital from the old one, so when we do eventually sell it, that's more in the retirement pot for us.

You will need an EIC from the Electrician (Electrical Instalation Certificate) He can issue that even if it is not all complete, he can issue it on the basis of all the circuits that are completed and tested so far, and then issue another one when the house is finally finished. Gas safe certificate if there is any gas, we had that just for the LPG hob fed from a pair of bottles. G3 unvented cylinder sign off The rest is down to the BC inspector. They will expect the building to be safe, stairs and handrails etc, Some will ask to witness a drain pressure test.

I am not suggesting changing the pitch. Lower BOTH ends of the rafters 100mm so the pitch is the same, then you can put 100mm insulation above the rafters and full fill the gap between them, finished tile level exactly the same. and inside plasterboard straight onto rafters, no insulation under them. This gets you more insulation into the same space, easier to detail and harder to get wrong, and no need for ventilating anything but the space between the external insulation and the tiles.

11mm OSB to support the UFH, 25mm battens following the joist lines (to allow you to notch the battens for the UFH pipes to pass) , 22mm chipboard floor = only 36mm higher finished floor than just the flooring straight onto the joists. The critical area will be internal door openings, frame those a bit higher. You only need to raise the whole 1st floor frame if you think losing 36mm headroom is going to make the ceilings too low. With a bit more work you could set the OSB as strips between the joists with their top level with the top of the joists so your floor is only then raised by 25mm.

I always set a cooker hood as high as possible so I can lean over a pan on the hob without banging my head (and it is not as though I am tall, far from it). The recommended height of some is ridiculously low. I had to modify ours to make it go higher that it's limited adjustment would allow as when initially set to it's highest setting I hit my head. So quite why you would want low cupboards above it beats me? Are you planning an extractor built into a cupboard?

Yes also in Scotland. We purchased our plot via an offer to buy subject to planning. Planning is granted and both parties are committed to the agreed offer price. Planning refused the deal is off. So I would start by agreeing the price with the land owner, get that offer in progress and conditionally accepted then bang in the planning application. I would still be looking to only put the buildings within the original house boundary but include the extra bits of field in the planning application as part of the garden.

If you are certain the land owner will sell, then I would include that (presently agricultural) land in the planning application (you don't need to own the land to do that) If the planning is granted to the plans you submit showing that land included then you have your change of use.

You just set the rafters 100mm lower at both ends so they are position for insulation above them rather than below them. The net effect is the same with finished roof surface in the same place. Assuming it has not been built yet.......

Alternatively make it a warm roof. Put insulation above the rafters rather than under them, then you can full fill the gap between rafters without needing a ventilation space.

I did similar in our bathroom and en-suite. I threaded the pipe through the web of the joists where they had to cross over. Even a very short loop in a small bathroom was a real mare of a job threading and re threading a long length of pipe through the joists. But it is doable, just, if you are patient. As noted do NOT even think about notching the top of posi joists. I would not even consider trying on a larger room e.g. a bedroom.

If you are trying to attach OSB to a metric frame on 400 or 600mm centres getting the imperial ones by mistake is VERY annoying. @epsilonGreedy is your frame built to imperial measurements?

BOTH sizes are available. But the ones I have been buying have been the metric 1200 by 2400 for some time, in fact when I mistakenly got an imperial sized one from a different merchant, it annoyed me I had to trim it down to size to match the others. just check the sizes before you buy.

I wired a house a while back where the ground workers had assumed a 100mm frame and built with 100mm block, but the frame arrived as 140mm. 15 years later the house has not fallen down yet.

Use 300mm EPS instead of the PIR (or have you bought that already?)

Can you post a link to the ones that do cost a fortune so we have a better idea what you are looking for. P.S don't copy that picture too literally, you can't have your sockets that low on a new build.

No problem if you use Appleby. Agreed some other makes are utter rubbish and not fit for purpose. A 35mm back box is perfect for a 25mm service void and 12.5mm plasterboard.

And how do they check you actually have an "electric car"? Will a Hybrid count?

Tempting. they are offering me 12.9p per kWh and 19p standing charge. But how long will they last before they go bust at that price and I am lumped on some random substitute supplier. And why does nothing like this ever come up on the price comparison sites? last time I looked there I was offered nothing under 16p

So yours combines the function of an MVHR as well passing the incoming air through a heat exchanger first? That makes a lot more sense and I can well imagine the exhaust air after the heat pump is indeed very cold indeed. the system described by the OP did not include an MVHR element and the incoming air to the rooms appears to be raw outside cold air.

WOW Who is offering 12.5p for all uses 24/7 in Scotland?

I am very skeptical of the principle of an EAHP. Yes it removes heat from the exhausted air, but that heat then has to be made up by the heating system as the incoming replacement air is at outside temperature. It kind of gets into the perpetual motion problem, unless in winter there is so much heat extracted from the exhaust air that it is leaving the building significantly colder than the outside temperature. It must work to some extent as some on here are using it and happy with it, but the amount of heat it can deliver is small. A conventional ASHP and mvhr is science that I understand and makes sense. Though retro fitting mvhr properly might be a lot of work.

I wonder if the new owner will join the forum?

The door opening in the frame is almost always over sized and the door liner will need packing. In a similar situation i used the absolute minimum packing on one side so there was more packing on the other side where the space for architrave was tight. Then i cut the architrave to size and fitted it. you hardly notice that it is thinner than the other side of the door.

I bought mine from Travis Perkins as strangely they gave the cheapest price. Delivered on their own wagon offloaded by it's HIAB.

Oak veneered MDF from Howdens, longer lengths than real solid oak, cheaper, won't warp or crack. Doors architrave and skirting treated with Osmo door oil: