Mr Punter

This plus plumbers and electricians not just mutilating the airtight layer. Unless you are doing MVHR with the associated install and maintenance issues I am not convinced that anything less than 3.0 is worthwhile, especially on a smaller property in the SE where mains gas is available.

The OP "wanted to gauge what target (and actual) people have had for air tightness in a traditional block and brick masonry build?"

I just had an air test done on brick and block flats and they were just under 4. No mastic or sealing as I did not want to get less than 3 and certainly did not want MVHR ducting.

It would not be an ideal first self build and unless you were skilled and experienced you could get your fingers badly burnt.

Turnkey including all fees, main contractor £1M, you as full time project manager £750k.

It will be much easier to tile a square one. You could get this in stainless steel and it may look better.

A good result. Did your builder use insulated cavity closers on the sides of openings for doors and windows, or close them with blockwork as per the drawings?

I think the VAT may be OK but you may lose your cil exemption. Bear in mind you may also need to gain approval from your insurers and mortgage company and you would need a cast iron guarantee that they would vacate and leave the property as they found it. For the sake of a months rent and the hassle of letting, combined with the double risk of the taxes it is probably best left empty.

I am no expert on these things, but from the photos, the welding is awful, some of the bearings look off centre but may be within tolerances and the top end of the pozis are awaiting some angled hangers to correctly secure them. Get your SE to look at this and give their professional opinion.

What you have done looks OK. The downpipe could be straighter and the gutters could be up a bit nearer the tiles and not tilting forward, but you are governed by the fascia.

Insuring unoccupied buildings subject to works is expensive as the risk is higher than a completed and occupied building. Sometimes you may not get all the cover you would like at a reasonable cost. I have a contractors all risk policy but it may not be suitable for your circumstances. Talk to a broker.

The raking section does not have to be belt and braces as it is protected by the eaves and gutter. You could use some self adhesive lead replacement covering right up to behind the fascia and down onto your new roof. Cut the whole thing carefully before you remove the backing paper.

I would stick with your original plan of stone cladding and render. Make sure the render is done correctly and the stone cladding is protected while render is being done. The stone may need some remedial work - like brick acid and / or pressure washer. I would avoid more timber as it is flammable and it can discolour.

It is quite surprising just how much a layer of 9mm OSB can stiffen and strengthen a stud wall.

If you have MVHR ducting, bear in mind that to feed in the ducts one end of the run must be clear of walls, so they sometimes need to be fed in before the walls progress. Strongboys and steels can get in the way as well, as can solid timber beams and flitch beams.

If the metal roof manufacturer is not willing to assist you may just need to make an educated guess. @saveasteading and @Gus Potter may give you a steer.

+1 I have used the cheaper Heritage range for a replacement at the rear in a conservation area. They look very good.

You are quite right. There is no minimum cill height for escape windows, just maximum. The windows would fail down on part K if you were making an application for Building Regs but if they have restrictors or a barrier they would be OK. WTF is this guy on about? Pathetic. In the past I have both added and removed door closers and added and removed window locks to satisfy Building Regs. Meanwhile at Grenfell it was considered fine to add a load of highly flammable material to the exterior of an otherwise fire resistant high rise block.

I can't see an issue here at all. Once the plaster is made good and the architrave on it will be fine.

Any gap would just be there for maintenance or ease of construction. You don't need 2 walls.

I would consider just treating it as a crawl space, closed off from the floor above. You could maybe put in a hatch for access.

What you have done looks fine. I give you my consent to stop digging.

Get in touch with the developer. They may help.

Yes, 12.5 will give 30 mins.

The guys that did the Hatton Garden heist managed. I would do some slots with a petrol disc cutter with a hose attached for dust and break out between them with a kango. Keep going until you achieve the right depth.