Redbeard

Fair point @joe90. I always try to go for excellent air-tightness inside *and* out - 2 bites of the cherry. When I was doing IWI NBT used to say the a/t layer was the final plaster layer. OK, but you deliberately sit the wood-fibre on a parge coat designed to make the wall air-tight, so why move the a/t layer at jcts? Hence I do both. (You have highlighted that I did not read the OP carefully enough, though. ;-( Re the original Q, with dpc only I'd worry about capillary action if you were relying on it for weather-tightness.

The dpc might provide a degree of weather sealing depending on whether it sits smoothly or gets trapped and twisted when the windows go in. It certainly won't do anything much for air-tightness. Compriband and/or FM330 or similar, and a/t tape.

If by 'gravel soakaway' you mean a French drain, they only really work with a perforated pipe at the bottom *and an exit*. A French drain without an exit is arguably a moat. Your text suggests a newly-laid path, but the pic suggests a not-newly-laid path. Can you clarify? How much have they raised the level?

Given the kit you are likely to have in a plant room I imagine there's no real need for fire-resistance, but if there were any risk I'd rate Fermacell over OSB. Have BC made any stipulations?

I take it there is plasterboard on the bathroom side of the 'tunnel'? Does it have a vapour control layer (VCL) on the cold side? If not, rip it out, install a really-well-taped VCL and re-plasterboard. Then build up from the cold side, filling any gaps with *air-tight* foam. Butt up tightly to any timbers, and at least one further layer over them. 150 - 200mm of PIR is not, in my view, too much. Keep all the joints tight, and tape them too, with decent air-tightness tape, not just foil tape. The existing does not seem to terminate very tightly at the top where it meets the rafters and the noggin. Make that bit better, and keep it all as tight as the btm of the proverbial duck.

Yes, it would mean you had wasted money on the Intello *if* you believe that several sheets of foil-backed plasterboard constitute an unbroken VCL. Since I don't believe it would anyway, stick with the Intello only.

I was very happy with Green Building Store windows and doors. 1st lot installed by GBS 10 years ago. Have not even thought of putting a paint-brush to them. 2nd lot there were a few logistical issues but I still like the windows! I have heard good things re Ecospheric, too.

Can't guess. Can you measure the gross ext wall area incl. openings and we can make an educated guess. If the original plaster is lime and is not covered in impermeable paint you can use that (augmented where it's 'not there' - between floors, behind skirtings etc). If there's any gypsum I would recommend that you take that off, and *then* do your lime parge coat. When I was doing WF IWI at 100mm it was North of £130/m2 incl basic joinery replacement, and that was quite a few years ago. The reason I say 'Gross' area is that working around and making good an 'interruption' can be as costly as fitting insulation to the same area.

I would suggest an 'Intelligent' membrane such as Pro Clima's 'Intello'. Expensive but good if it does what it says on the tin - and I have worked on the basis that it does!

How high - up the the roughcast? And replace with something else or leave it exposed? And what about the render beneath the crack/DPC? Sorry, I meant 'hack off the render *up to* just above the crack'.

Contrary to what the Building Regs would have you believe (DPC should be min 150 above ext gr level) rain in some parts of the country easily bounces up 300mm, particularly off hard surfaces. If I am 'reading' correctly 300mm is about the level of that waste pipe. I had taken the internal floor level to be below that, but is it? Or is it way up by the stepped-out roughcast?

Looks too hard and too moist. If the crack is the dpc, I would hack off the render just above that level, and also look at 'softening' the horizontal surface. I take your point: ...but somehow you need to try to achieve some 'softening'.

London (yellow) brick? Possibly the right sort of area. I could not 'aesthetic' my way out of a paper bag but that on the lower elevs with something darker above seems nice-ish. I'd be worried about white brick.

I have not had a comprehensive read, so apologies if I am countering anything you have already said, but I assume your roof is finished, with tiles/slates/whatever already on. If not, can you increase the depth of rigid WF? As per others' comments it looks like, strictly, your roof is 'the wrong way round', in that, crudely, the 'conventional wisdom' is that you have more above the rafters than between (66.6%/33.4% is sometimes suggested). However I have seen some details (sorry, cannot remember where, but I have a feeling a colleague did this) using WF sarking and fulll-fill Warmcel (recycled newspaper) in deep I-beams, so clearly some people have different ideas. If you decide to 'go for it', why not just put counter-battens on under the rafters to achieve the depth you need, so you do rafter depth in one direction and between counter-battens in the other, which has the virtue of each layer covering the last layer's gaps?

Welcome. What are we looking at? Just the single-storey side bit, or is the front 'projection' part of the proposed ext'n?

I may be misunderstanding this, but I take it this is 1 storey only. What is your absolute limit on internal ceiling ('roof') height? If you build it as you suggest but with a mezzanine sleeping platform could that make it more workable for year+? How are you proposing to build it?

If that's the case the question then is 'where do you run the cables now?' I don't know what sort of house you have, but is a service void feasible? If it is, and you get the spark to pull all the cables, you'll still have to seal all the holes in the membrane, but you can then run all the cables *inside* the a/t layer, except those few which have to breach it, and you now know how to do even that with fewest risks.

Yes, my suggestion of 'grommets-with-a-slit' was effectively to give a substrate to work off, similar (but not as strong as) @Iceverge's ply. Each 'cut grommet-hole' would still require Tescon tape or butyl tape to complete the job.

My thoughts entirely. I am assuming the wiring is pretty well done and that it is far too late to disconnect cables and run them thro' grommets as should have been done. Therefore, as per @Iceverge's mantra above, separate out those bunches, get some OSB/ply behind if you can (as it's a darn sight easier to stick to something which doesn't move away from you!) and do the 'split grommet'/tape idea which I described above. *for each separate cable*. I have never used Pro Clima's butyl but still have some Pavatex butyl tape I use when necessary. It takes bends better than standard a/t tape, though Tescon ('cloth') tape is pretty forgiving. It is very tedious but do not despair. By the time you have 50% of those holes done it will feel like it's all downhill (in a positive way!) from there. And +1 to the DIY 'blower door' idea. That's on my list of things to make too. Best of luck.

You can make your own. I would not like to guarantee they would be quite as good as the proprietary ones, but I have had some success. Basically EPDM, as many holes as you need, and air-tightness tape around the perimeter.

Oh dear! You are not alone, though. Ideally the spark would have used purpose-made cable grommets. Would you have room to separate the cables and cut a square out of the membrane? Get a piece of EPDM (rubber roofing) bigger all round than the square you have cut out and drill holes a good distance apart for the number of cables you have, each hole slightly undersized for the cable. Then make a cut from each hole to the edges of the EPDM square and tuck the cables in. Then get short lengths of air-tightness tape and tape round each cable to the membrane. Finally, use air-tightness tape to tape the EPDM square into the hole in the membrane.

I may be misunderstanding the pics but pics 1 and 4 seem to indicate some 'local issues' more than just the beads. If it were me I would perhaps be allowing in my costings for getting the suspect areas (in particular pic 1 by the door) chopped out and re-rendered as well as the beads. As @nod suggests, use plastic or stainless beads.

Not the same thing at all in terms of aesthetics, but you might have a look at Green Building Company's Stirley Barn project.https://www.greenbuildingstore.co.uk/technical-resource/cre8-barn-stirley-farm-enerphit/

I mean weep-holes to allow water drainage from the retained land behind to reduce pressure on the wall.

It's not quite an answer to your question, but am I right that there are no weep-holes?