Mr Punter

Is this a painted finish? Some of the cement boards are very resilient and rot proof.

Unbelievable! No wonder this country is falling behind. Can't you dose her with steroids? It worked for Bulgarian shot-putters a few years ago.

There is a Corten clad house near me. I know the contractor who clad it. Apparently it is very difficult to work with and getting your seamless junctions to floor, ceiling and windows will be very hard to achieve.

I once had an issue on a house where the electrician would get a shock when he touched a wall light on the stairs. It turned out that the wall light was fine but he had one foot on the stainless handrail to keep his balance. The handrail installer had used a very long screw and this made the handrail live. No issue until he provided a path to earth via the light fitting.

You may be better off having a full coping at the end and a partial one or two elsewhere in the run, especially if the cut would be less than 2/3.

I am laying the DPM on top of the concrete slab instead of under it, just using extra wide DPC for the blockwork so we have a decent lap. I have not done it this way before but it makes sense as it would be awkward to lay before concreting.

I have only done one of these. It was a small house but still on the limit of what the unit (Vent Axia Sentinel Kinetic) could manage. Maybe your place would work well with 2 units and fewer steelwork clashes? Obviously the more restricted the ducting, the higher the pressure needed to push the air through, the harder the units need to work and the noisier the system. You will need to agree any steel necessary penetrations with the SE and get these made during fabrication, rather than on site. Do you have a single design / manufacture / install contract for the superstructure?

If your cold water pressure and flow is OK, the free combi would be tempting and as long as it doesn't cost too much to fit. You could always change later. Unvented cylinder and system boiler would be more standard for your house.

I don't like insulated plasterboard. It is expensive and impossible to recycle and you will need some very long screws to fix it unless you stick it on. Have you worked out the comparable buildup thicknesses, u values and material costs? It may be worth having a play with. You could look at slightly thicker pir on the wall and just use 25mm battens, which still gives enough space for electrics and pipes.

Without wishing to give you more work, is your floor insulated? If it is just solid concrete you could add some insulation to both the floor and ceiling. 50mm Celotex on both would make a huge difference.

If this is a proper building needing building regs I think you may struggle. Blocks without foundations would be a no-no. You can't have your timber frame walls starting less than 150mm above ground. You may do better going for a system that is already in place for the type of building you propose. You may notice that most of the ground screw solutions are for "garden room", "home office", "decking" and so on. No good for a proper habitable room under building regs.

It doesn't matter if the concrete has completely dried. Your tiler may even be up for laying the tiles into a thick bed of sand / cement and forgoing the concrete.

90mm is a fair amount. Concrete may be best. Yes, it needs to cure / dry before tiling. Another option is the pedestals but they may be more problematic / expensive.

With sand and cement the membrane is not an issue. The liquid screeds can get through any small gap and mess up the job.

Have a look at the Guttercrest ali stuff. There is a bit of drilling, cutting and riveting involved in the install but all doable. I found zinc is easy to damage.

They love a service contract in the fire industry. Alarms, sprinklers, extinguishers, detectors. Makes a nice regular income.

Plastic is easy enough. Some of the metal stuff requires some skill at sheet metalworking. I have done ali and some lindab steel. I cut the ali on a chop saw but I rather butchered the Lindab. No rust running down the place 15 years on, so I guess it was not a disaster. Zinc and stainless will be expensive.

Have a chat to your sparky as they can be a bit sniffy about cables in external cavity walls.

That gives you a fall of about 1:40. Even if your invert at the exit from the hut is 250mm because you want the pipe buried, you still have 1:80 which is just about acceptable.

That sounds really strong. If using all-in ballast that would be 1:3. Often for general purpose concrete I mix 1:6 cement:ballast.

Sorry, I missed the woodfibre insulation detail but you mentioned XPS insulation. Is it OK for use woodfibre below ground as per your detail? Best have the manufacturer and BBA certify this as you won't be taking it out later! As for the cavity below ground, again look at the BBA certificate and the manufacturer's instructions and warranty. Apparently Siberian Larch is in short supply and the local stuff is nothing like as durable. The breather membrane over the top stone will need to be supported. Are you having a flashing from the insulation to the stone plinth to prevent rainwater running into the cavity? It is worth making sure the door threshold and reveal details are nailed down.

Ideal Combi and Rationel are part of the same group. Installation is often a downfall and on the video above I think many of the issues would not have arisen had the units been correctly installed. That said, I had an issue with condensation on a Velfac window some years ago and in spite of the warranty Velfac were keen to blame the install (which we did ourselves). The homeowner was not taking any nonsense so eventually Velfac capitulated and supplied and fitted a new one at their own expense.

It may be worth settling on a profile manufacturer and profile you like available in the colour you are after, then it will be easier to compare. I guess most fabricators can do supply only. I have used online for composite doors and they were OK.

I am a bit uncomfortable in having flammable ventilated rainscreen cladding with plastic insulation and membrane. Hold a flame to each of these and see if they burn. I once had a house catch fire when a plumber was joining some copper pipe for the gas and the flame from the blowtorch set the membrane alight. Luckily we managed to get access to the top and got a hosepipe in there before the fire took hold of the building.