Mr Punter

Another vote here for Karndean / Amitico / LVT.

Yes, if you use the plans from your neighbour they will not meet regs. There is not enough ceiling height and not enough insulation in the roof.

I am not sure where the manhole is in relation to your proposed new connection but if you can get to the right invert level and are not in completely the opposite direction to the run you should be able to use a slipper bend to the manhole and redo the benching. If you don't know what you are doing, it may be easier to just get a price from someone who does. I would not do this job myself but I sort of know how i it should look.

I do my own but just use a drawing program (CorelDraw). Paper and ruler also works!

I think your "rainwater pipe" could be gas ducting!

Do you have consent to build over the sewer?

Your pipe will need to come in at the same level as the other side branch. You cannot connect higher up the side of the chamber.

I think you may be OK as this was not caused by incomplete construction.

Just to cause further confusion you could consider a cold roof with (from outside) torch on felt, 18mm OSB onto firrings minimum 40mm as ventilation, fully filling the rafters, 50mm PIR under rafter joined with ali tape, polythene and plasterboard.

We have used the membrane with scouring pad / pubic hair topping like @Russell griffiths used. I think it means that you don't get a puddle of condensation directly on the zinc. I think the risk is reduced with some of the zincs with pre-patinated finish.

We had a flood in a finished mid terrace show home. Fitting popped on the first floor and water was on overnight, so I came and the downstairs was like a rainforest. We managed to dry it out with heating on full plus big radiant heaters and 2 big dehumidifiers. We left it all on full tilt for about 3 weeks. It was fairly dry after week 1 but there were 2 layers of plasterboard and some mineral wool in the ceilings. We kept pretty much everything and redecorated. The ground floor was concrete and tiled. Just make sure you use loads of heat and dehumidifiers.

Bear in mind that for each 100mm you excavate you will generate about 15 tonnes of material. Maybe you could get planning consent for a more attractive building with pitched roof? You could overlay the worst case PD scheme in elevation as a contrast.

I have found that it varies a lot between projects and BCOs. Normally I would book a visit before pouring foundations, ground floor walls up to DPC, structure complete pre plaster, and completion. At the end of the visit I suggest the stage when they would like to reinspect and if they agree, fine and if not I ask them to specify. Sometimes they will want to reinspect stuff they picked up on but are normally happy with pics and email. I have never done a progress report to them, although I sometimes get an email from them or warranty providers asking where we are up to. Two and a half years and six staff and you are not done yet. It seems a lot of work for them when you look at their fees.

Is there a reason you cannot have a pitched roof?

I got a small brain haemorrhage reading the first bit of the original post. I dearly hope that zoot is not behind this. Can I confirm the TLDR as "I knowingly bought a bit of a shitter and am now having a moan about tradesmen who I have hired to sort it"?

You will just need to make sure it complies. Is it above or below the beam? Are there any openings above where it exits? You will also need an extract fan in the WC and there are regs regarding the distance between extract and flue terminals. There may be another duct from the cooker hood. Better get it all settled at the design stage.

A small point is that if it is a combustion boiler it will need a flue to outside air and the whole length of the flue must be accessible for inspection. Also you need below ground drainage for both the WC and the kitchen sink and nothing is indicated. The plan does not work for me.

It would be helpful to see the existing ground floor plan. Just a plan view with stairs drawn on. There may be a simple solution.

Some little drill holes. I can't think what else they could be for? House looks fine though.

I just use a spade and hand dig trial holes myself.

Make sure you leave enough space around the unit to get to the filters. Tomorrow afternoon I have the fun task of climbing around a very tight loft to swap out the filters in a Vent Axia Sentinel Kinetic plus, followed by swapping out a sewage pump in a mini pump station in the same house. Glamorous job being a property developer!

Was this the inspiration behind the wiring?

Looks like they have some injected damp proof stuff into the bricks as well.

Agree with @Temp and @joe90. The neighbours would not work with current building regs. You need at least another 100mm height. Have you seen the neighbours?

I have used some slate tiles in this situation. They measured about 300mm long by 60mm high by 10mm thick. Butted up with no grout.