Mr Punter

The Hardie Plank / Marley / Cembrit type boards are fire resistant and easy to DIY. About £25 per sqm.

If your plan is to perhaps have this as a future habitable annex, place the entrance and windows so it suits the future requirement as well as current. I can't see how a 100mm block wall is going to be "nice and warm". You may do better having timber framed walls but you will need non combustible cladding as you are near the boundary. You may get a small timber frame company to fabricate and deliver the walls and soleplates and supply the roof materials. They will often get better terms than you will get from a builders merchant.

I don't think this can be so. I cannot see how a covenant can prevent land from being owned by, for example, the state or a company.

Although you say otherwise, I think you should include provision for electrics, water and drainage, so you can have a WC. Quite annoying if you have to traipse over to the house and back when it is raining. A floor plan may be useful. Maybe don't go for the windows to the floor as they may restrict future kitchenette placement. Rendered block may be cheapest.

The covenant refers to use, not ownership.

If they want to make this an HMO or hostel they will need planning consent.

I have only taken these out twice. Once on a plot that had outline pp and an original developer covenant from the 1950s and once on a plot that could have been within a chancel repair area. The fact that there were no objections to the pp from the 1950s developer meant the premium was negligible.

It is often fairly simple to obtain insurance against the enforcement of covenants, chancel repair liabilities etc. and the premium should not be large.

Velfac do something like this where the sliding leaf is initially in line with the fixed pane but can be pushed outwards to slide past.

I liked that Combimate guarantee "when used with Combiphos, that if the unit it is protecting experiences significant scaling, we will contribute up to £500 towards the cost of repair or replacement." As they are part of Cistermiser I think that the guarantee is worth something.

An interesting discussion in an product area full of half truths and pseudo science. I have installed the electro magnet type and recently had some Combimates installed on recommendation from @richi. It does look as if the claims that @Polly makes regarding product uniqueness in energy use and non-release of chemicals may be hard to justify but I would be interested to see any actual scientific research on the reduction in limescale on boilers, kettles etc.

I am a big fan of contractors who come up with a solution rather than just the standard shrug of shoulders. Pouring and curing a concrete slab is a hell of a task and lots can go wrong - deliveries, mix, weather, poor prep, poor site operative, broken equipment. On most housebuilder sites nobody cares, unless it is civil engineering, in which case it is all inspected, cube tested etc. A few years ago near Brighton I watched a reinforced concrete bridge being built, then a couple of weeks later the main supports were torn down by a huge demolition excavator. Apparently the concrete supplier got the mix wrong. Mucho £££

Lift and slide seems to be the consensus way forward for airtight and operation.

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.