Mr Punter

If ground conditions allow, I like the concept of insulated slab foundation.

Leave white grout for a while and at best it will look like you piss in the shower (which I am sure you don't).

If it was me and planning consent is not needed I would not involve BC. I would want to dig down to a reliable base for my foundations though. Deeper foundations cost a fair bit in muckaway and concrete, so if it is only for an annex I would do as little as pos. When / if you come to sell you can do the "purchaser must rely on their own enquiries" as to whether BC was required / applied for. Mortgage lender will only be interested in the main house as security.

If it is the cold bridge and condensation you need to remedy with insulation and vapour control. The plasterboard is damaged beyond repair, so you may as well remove it. I am surprised not to see damp on the vertical board that is on the left but there may be a void between the board and the flange, so perhaps the warm moist air enters the void, condenses on the flange / upstand and runs down to collect on the ceiling.

It looks as if you have a warm roof with the vcl and insulation above the OSB. I think you have correctly identified the issue and the rooflight upstands are creating a thermal bridge. What is the thing in the puddle?

No they will not care. There will be X years remaining of a 10 year warranty. They will not be checking completion date, building regs final date etc, just that if it was built within the last 10 years it has a warranty.

On many of these policies you may be liable for the first 2 years, so it may not matter.

It seems that you have eliminated weather ingress and so this is condensation. Could you remove an area of damaged plasterboard to expose the rooflight / roof interface, roof structure, insulation, vcl (if fitted) etc.?

The water damage is either weather ingress or condensation. If it is weather, you need to look at the roof and rooflights. If condensation you need to look at insulation and vapour barrier. I don't think your heating and ventilation systems are to blame. Where is the house located?

Does it mean "covfefe"?

As it works without the durgo, have you looked at swapping it for another brand? Try a https://www.screwfix.com/p/floplast-af32w-air-admittance-valve/42968 and just take it back if it doesn't work out.

Unable to attend but I hope it goes well.

I am also a fan of combis and was explaining why I did not fit one. Your advice was much appreciated. There is currently no timer on the immersion so the proposed test would be tricky. Cooking is electric oven and gas hob. There are 4 in the household. From what you say about the downside of leaving the boiler switched off, I will try the twice daily blast on the boiler if I can get the timer to do what I want it to.

The boiler is on level 4 in a roofspace plant room, with kitchen on level 3, bathrooms on levels 1 and 2. The hot water tank is on level 1. The only other option for the boiler was level 0 and I did not want to wait an age for hot water.

I just had a look and it is a slimline Joule Cyclone Indirect. Only C rated, but it needed to be shoehorned into a small cupboard.

The regs are different in Wales. For example they require sprinklers to meet Part B. Not sure about foundations though.

In answer to the OP, if the plot is near your current house then keeping the house and just moving once would be a lot less disruptive and may even be cheaper if you can get the figures to work. One potential issue is that you may finish the build and are not able to sell the current house quickly / for as much as you wanted.

I know it is off topic, but can cooling the floor cause condensation?

I put it down to the hot weather.

I think 2700 to 3000 if you have the height. I prefer boarded ceiling first, then walls. Like others have said allow about 10mm at the bottom of the board so if you have a leak it does not wick. The 3000 boards are a pain to heft about. You may do better with 2400 and a cut. More tricky to plaster and decorate but far better than 2400.

You could probably get the site survey and drawings done cheaper if you shop around. If you like the type of designs the architect produces, go for stages 2 and 3 only. Just get them to do the planning drawings.

The membrane was supplied and installed by the zinc contractor. It seemed strong and the curly plastic strand layer supported the zinc about 8mm above the membrane. I am not sure what happens if the zinc is left in water but I assume not good. I can't recall the name of the membrane.

I have used a membrane underneath zinc that has a plastic layer on top a bit like Scotchbrite so if there is condensation under the zinc it won't sit in a puddle.

Great looking job. If it sinks is that down to you - as you did the base - or them?

I think that ANYTHING is better than a macerator / Saniflo. If you can get this to work with proper gravity drainage, do it. You will not regret. If the pipe is exposed to the sun you should use the black or grey stuff. If it is underground or covered use brown, but ANYTHING is better than what you have (other than crapping in your own pants).