Iceverge

I would consider omitting the top layer of OSB entirely. It's doing nothing really. I would just diligently tape the joints in the ply too and forget about the vapour membrane. You could spec the trusses with a wider top chord from the Factory to avoid the need for the additional batten. In any case I would use 2x layers of 100mm PIR, joints staggered foamed and taped. Don't fret about thermally broken fasteners. They'll be almost zero performance difference to a metal one as the cross sectional area is so small over 200mm. The counter battens and batten is good for tiles but for metal cladding you could save some work and just put a 45deg (allowing for drainage) 70*20mm batten instead. The recess in the corrugation making up the rest of the ventilation. ( I personally think the corrugations alone are enough.....) Have you a need for overhangs and eaves? Do you have a detail for these?

The whole thing is worded very much like " you really should pay us to do it properly" while stopping short of mandating it. I can't see anything to stop you making a reasoned assessment of the criteria and self installing. I also don't think a good quality single skin plastic tank is much more risky than a double skinned one provided steps are taken to prevent it from external damage. Even an appropriate paint should keep the UV away.

I think single skin is ok if you're more than 10m from a watercourses or 50m from a bore hole. Paving slabs or concrete seem to be suggested though. https://www.oftec.org/consumers/off-gas-grid-guides/home-guide-to-domestic-liquid-fuel-storage-up-to-3500-litres

How stuck are these windows? Stuck on both houses? How about your ultimate neighbours to the very south, can you make more of the south facing glazing here? Maybe a deidentified aerial pic would help. I under stand the desire to keep everything standard but as you will actually be living in one house I would consider making that one as nice as I could by maximising the use of the sun. I would certainly include a walk in shower on the ground floor and a room that could be a bedroom too. The walls appear a little skinny. I would get them to at least 360mm to stand a chance at getting good insulation with a timber frame. 400mm if masonry. work on 130mm internal walls as a minimum rather than 100mm. For me 4 x W/C is an overkill for the size of house. I would do 1 x ensuite, 1 x family bath and 1 x ground floor showerroom. An option might be to move these to the north side of the house and use frosted glass and/ro roof lights to avoid the overlooking issue. Perhaps a pic of the street scape too if you have one might help.

Is this for selling onwards or for occupation? Where's north please?

Yes so long as you make sure to very diligently cover all the ducting with mineral wool and tape all penetrations of the airtightness layer

Can you not lower the wall plate?

Raise the tie for the rafters, add 125mm PIR on the underside of the sloped sections, Full fill the rafters with mineral wool/cellulose and pur 400mm above the flat section. OSB above the rafters and batten and counter batten above for ventilation. Add a 100mm service cavity below the rafter tie for MVHR. Forget about any storage in the roof. It's simply much too small. Even with your original design it's only 1000mm high at the apex.

Out of interest @JohnMo, given that you were aiming for a higher level of performance would you do the kit again or make something up yourself?

It'll be fine if it doesn't tear. I had some Protect VP400 exposed for about a year on my garage wall.

The feel of how high a ceiling has to do with the ratio of the floor area to the ceiling height. Our 2.4m high utility (2.6mx3.3m) feels much taller than our 2.7m bedroom (5mx4.5m) so I wouldn't worry in your case. Nothing to stop you installing slim profile surface mounted downlighters. A wire with some acoustic sealant poking through the plasterboard is much less of a fire/noise risk than one punched through. Someone did a mass Vs cost chart ages ago. Standard plasterboard came out on top for noise. I'd be really careful about noise transfer on this one. Washing machines make themselves known.

Give me a cross section please of the house. I think I have an economical solution in mind.

If you do want a warm attic I would move the airtighess layer to above the rafters as a taped sheet of OSB that wrapped around the rafter ends and through the wall plate to the inner wall. Have you a plan view or your roof and a bigger cross section?

A 25 deg roof will be near impossible to insulate correctly to the eaves and airtighess will be a mare too. As @ADLIan says you could do a cold attic. You could still store stuff there. I would put an airtighess membrane and then a 20*70mm battened service cavity below the joists before a double layer of plasterboard. The MVHR could penetrate the membrane but you'd need to be diligent with taping the penetrations. Mineral wool or preferably cellulose over the top.

What's in the room above? For noise reasons ( washer dryer boiler) I would consider double boarding it and getting rid of the downlighters.