Mr Punter

If they come back with a defence it would be worth you downloading some design details from other suppliers or walk on rooflights where their drawings concur with the way you measured. If more than one firm has your method as a standard, I would think your chances of of success would be greatly strengthened. You will probably need to submit these to the Court and the other side before the hearing. Also, did you get quotes from other firms and if so, what was the basis of their offer(s)? Again, if you can establish that everyone else used dimension X and the other side seemed to use dimension Y I think your case will be strengthened, particularly as you are an individual consumer so there is no presumption that you should have known better.

Agree with @Russell griffiths that 1200 is minimum. Any eaves / gutter overhang or is it a gable?

A knowledge of the ground conditions is a must. Also, is the basement far from boundaries, which will make excavation simpler? What is it to be used for? Does it need drainage for WC etc? Any light wells? If you look at the construction cost per square metre of basement being 1.5 times that for above ground and factor in that the end value will likely be 70% of the above ground, does it look viable?

I don't think it looks attractive. What is behind the design? Is it to follow a local vernacular or a certain style or look that you aim to achieve? You will be spending a lot of money and time building this, so it will be worth paying to get a couple of professionals to come up with some ideas, as you risk it looking a bit "nothing". As with @ProDave the dormers are a no-no, as is the arched window and the Georgian windows. The outside needs to be properly resolved.

Unless you are building over a swamp you are being paranoid unnecessarily cautious.

As long as it has manufacturers accreditation (BBA etc) for this there is no problem.

Do it from the inside later.

Concreting looks straightforward but is one of the most exacting things on site. It is very easy to mess up and timing is crucial. Check out @PeterStarck's blog https://forum.buildhub.org.uk/blogs/entry/73-now-for-the-foundations/ Most sites the contractor would get away with this and it is probably only becase Peter was meticulous that he didn't get flannelled into accepting a cock-up. Good to see you checking levels at each stage - not just assuming that the next layer will compensate. Give yourself a pat on the back. A fantastic job.

And in the rooms...

Some pics

I would go for running 2 anywhere you are likely to use a tv, computer, printer or phone. Run back to a central location where you also have power. I terminated them into a couple of 8 port Trendnet patch panels flush mounted into the plasterboard and connected up with patch leads into a 16 port gigabyte switch. I went for a fanless one as it is a wardrobe and I didn't want any noise. Also ran all the tv coax to the same location. It is a bit of a cable fest, but it works well.

It would be up to you neighbour if they choose to consent to you building on their land. You have shown this as a solid wall but it will probably be a cavity wall so may step in a bit more. You would also need their consent and an easement if you have any gutters etc. I don't know how option 2 would work as you would need your roof to cover the new wall, unless it is built as a parapet.

Standard for plumbers, I am afraid. They like to run hot and cold services next to each other with no insulation because it is quicker and they don't give a **** about whether you have to run the tap for a minute to get water at the correct temperature. I am sure there are exceptions but none have worked on my sites.

We had some concrete nice and level but not as flat as I wanted. I paid for a supply and place liquid levelling compound supposed to be good for 2-10mm. Ended up worse for level than just the concrete, so sent them packing after 50% complete. I think they were used to doing Gyvlon at 50mm, which requires little skill.

I would lose the render and replace with granite, but that could be a budget issue. Failing that, natural slate or zinc, but it will look rather dark overall.

I would go with the VAT company. You will also need to register for CIS and be aware that Health and Safety requirements can be more onerous. Regarding finance could you get a Buy To Let on your current house?

Yes the beamshield stuff is sculpted to fit the beams snugly. We had the Plus version and it wraps under the beams as well. Fairly solid to walk on. We had very wide beams and there was a fair amount for the insulation to bear on. @Russell griffiths DIY option is not a bad shout though... Maybe reduce the insulation width to, say, 400mm, as I think it would be strong enough and fit with sheet widths. Once you have the top 50mm, it will be quite solid.

I have had Beamshield supplied and fitted by Charcon. They supplied the beams, craned them in and installed the polystyrene. My only critisism is that it took ages to get Sprigvale to pick up the surplus polystyrene but I think this is an industry wide issue as it is expensive to transport to a recycling facility. We got another firm to do 75mm concrete and mesh. You can tell the supplier what loadings you want it for. We wanted to build blockwork ground floor walls and got a spec accordingly. You would not know it from a solid concrete floor. Cost was £17,300 for the Beamshield and £5,700 for the concreting about 340m2, so £68 per metre. 2 years ago. Probably more for a smaller area.

Worth bearing in mind that over that period house price inflation was 220%. Did you get the land "free"?

I have had a staircase going from a living room and I would not repeat it. Noise / smells etc. An en-suite upstairs may be good. I can't see fridge / freezer / oven / microwave / dishwasher in the kitchen and there is not much storage there. Woodburners can quickly overheat a well insulated space.

I am pleased they don't do a soap dispenser version...

Can they not use aluminium cable?

Do the gas network have any easement or wayleave? When you bought the property were you aware of the gas main on your land? Service plans are rarely very accurate. If it is shown on your land the location can be very approximate and as you say, it is easier to soft dig than to remove / replace flagstones. If you want it done quickly, contact the emergency number and say your groundworker has just hit a pipe and you think you can smell gas.

Unless this is 4 storey plus, brick / cavity / block construction gives adequate fire resistance. The cavity is not strongly ventilated enough for the chimney effect. The weepholes may allow an external fire into the cavity, but the building would likely be compromised by then in any case.

OK Consider ripping out all the timber and sleeper walls, compact the sub base, stone / crushed concrete. Insulation then concrete or concrete, insulation, screed or chipboard.