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I thought 25mm was the minimum for roofing anyway, wouldn't use less

Tektite push fit copper might be a decent alternative - slimmer than plastic push-fit externally and wider internally, no need to hire / buy tools versus press fit (which Tektite sprint looks similar to assembled) Working on our own house they're my preferred option if I do need joints in copper but generally I prefer long runs in Hep2o terminating to copper only as needed.

Bit surprised with the focus on fire safety you'd get any concession on this one. Maybe they thought you'd slim down sweating while waiting to escape? 50mm is more than a fraction, I think you might struggle to get that past a BCO. There's no point being able to climb halfway out of a window but jammed on the cill. Maybe if it's picked up on (or you're on the larger side anyway) you can look at whether it's easy to make the stairs a protected corridor instead?

For a door handle, plug, switch etc I'd think in terms of swapping one out from the least used room rather than preemptively buy spares. Kitchen door/drawer is probably the one that worries me a bit and if I was having a custom colour or finish would worry more. Expensive to buy just for spares though. It's always reassuring to have a couple of full tiles or flooring planks left over, but generally from a planned %wastage rather than buying a whole extra pack for spares

Plastic lift and slide like Aluplast will probably be able to handle that size / height and have decent u-values but may not be any cheaper than the popular aluminium systems and will definitely have bigger profiles.

Regs are pretty strict about tundish discharge. You definitely can't run in hep2o or similar and I think your tank size limits whether you can discharge into soil stack. We ended up with 35mm copper for a similar length run as you have to size up depending on length and the number of bends

In real life don't put your toilet that close to the wall, next time you're sat on your throne enjoying a quiet moment of reflection pay attention to how much space you actually need at the side.

On the inner leaf of the new wall - if you want something to show your builder then LABC publish renovation guidance that covers this including a diagram on page 14...

Personally I'd worry at least as much about leaks around the rest of the home. Drain with a waterless trap sounds good here but there are smart leak detectors and remote shut off valves that are probably a better fit in most cases

Honestly that will be the least of your issues compared to the effort of cutting rigid insulation to retrofit tightly between old joists. Much easier to do something such as @JohnMo's suggestion. Pretty sure this will fall under building control renovation of a thermal element.

Congratulations on a lovely build, I'm jealous of how well you've kept to schedule! I think this is a really good way to reflect on costs - you've achieved a better house, got exactly what you want, all without paying over the odds - plus you're lucky enough to have enjoyed the self build experience along the way.

Most people chipping in would lean towards more space kitchen side based on lived experience and so maybe future buyers will think the same, but at the end of the day, you're much better placed than us to know how you'll use the space and you've clearly designed for the limited space with your lift up cupboard doors and lots of drawers, so (as exhaustingly ever with our own projects) you have to trust your own decision making. One possible approach would've been to tile through, not fix down the island and see how you live with it but I think that ship's sailed. Very nice kitchen by the way!

Having seen the photos, I'd personally want to compromise on the dining side to give more space kitchen side now I see it's the main working space. A better trade off might be table width 800, or you can probably reduce walkway to 1000, probably reduce the chair space to 200 when pushed in, consider banquette dining seating to save around 150 - basically pick options and get to say 1050 for kitchen (our last house, wouldn't have wanted less - you'll wait or push past not walk around the island in real life), assuming it didn't impact the living space far end too much

@Super_Paulie yes that's great for working space - looks like your pinch point of island and worktop corners is an example of what I mean? Quite a bit narrower but only used for passing through.

If it's a secondary walkway with an occasionally used cupboard and having an island versus say peninsula is important to you then down to about 800 is workable I'd say, as long as there's better access around the rest.

The fixed scope and the background of that scope being defined in the context of costly issues with your previous roof may well weigh in your favour in any action, but there'll still be risk in any court action. Have you checked whether you have legal cover in your home insurance or self build insurance? If so, they may offer advice.

Yes raise the issue, 22mm is a lot to be out by DPC. What were the foundations like? Has he created a problem or inherited one (even so he should've been picking this up)? If you'll have level thresholds at multiple openings over that length then one potential problem might be you'd struggle later with a liquid screed. Our brickie had to pick one corner up over a few courses, marking the spacings for the next few courses out on the corner profiles seemed to work well (say the good end is 75/150/225/300 then the low end mark 77/154/231/308 etc) until you're back level. You have to have an honest discussion, ask how he plans to get back to level, suggest taking down a course or two as the lower you start to fix this, the easier it'll be. Give him a chance and then decide from his response (both attitude and actions) if he's someone you can work with or will be battling with for the rest of the brickwork. When you say soil pipe into the back inlet, I guess you only mean grey water, not a toilet?

I'd be getting in touch with your tilers before doing any remedial work. If you're having problems already it's hard to believe they won't get worse, put the onus on them to fix any issues

Have you considered swapping the toilet and sink positions? Shorter soil run, keeps toilet tucked away, better access to stuff on shelves when at the sink. Maybe run 32mm basin waste into the 50mm shower waste for one less connection at the stack. Personally I'd lean towards a rectangular shower following the room shape.

That sounds wrong to me - we considered their T1 and I thought a specific benefit was no need for slate and a half. We also looked at Klober dry line S but didn't go for that due to (possible unwarranted) concerns whether it might fade or bow in hot sun. In the end we oversailed the slates about 50mm past the mortar pointing to create a thin edge profile (needs slate and a half or large slates if at all exposed) but I suspect that only really works where you've brick rather than fascia board below.

This Passive House Trust article makes the case that:

Have had similar and I'm pretty sure that the caps on each side slide off sideways to reveal a screw below. You'll probably need to insert a thin flat screwdriver or blade to prise the joint open as they click into place when fitting.

What if any surface layer will the drive have? If that will be impermeable anyway (e.g. traditional tarmac) then there's no point changing to a permeable sub base. Can the drive be laid to a slight fall, such that run off is naturally towards a permeable area (soil) on your own property? Looks like 100m2 of drive needs 20m2 of permeable run off area. Otherwise you're into needing to construct some sort of drainage/soakaway. I'd work out how much MOT type 3 you'd need as a replacement sub base to at least see where you are on cost and consider how that compares to any alternative approaches. @Mr Punter you're probably right and no-one may check, but if they do then costs to redo a finished drive later will be a lot higher than cost of redoing just the sub base now.

Welcome and good luck! These comments sound a lot like us, so fair warning that along with excitement you'll have some challenging times ahead, especially managing the gaps between how things are meant to be done and how they're actually done on site. As soon as you're aiming for something better than the average new build quality, you're asking trades to step out of their familiar comfort zone - some will be up for that and others will just do what they always have, regardless of any construction detail you supply. Managing needs a lot of time on site to pick up on any issues before they're too built in to easily or cheaply correct and also includes being responsible for a million small things that fall between the main trades but need to be done, plus a lot of time off site making sure you'll have the materials ready at the right time and sensible cost. So, an exciting journey but be sure you know what level of commitment you're taking on.

Endless amounts of dust 😞 Now you've done the hard part, I wouldn't leave it too long before foam, tape and boarding over - you'll be surprised how quickly the insulation will bow now the foil has been removed from one side. (Too late for you, sorry, but I'd probably have packed out the timbers with say 12mm ply strips and used longer plasterboard screws to avoid having to cut back the board depth)