Mr Punter

It is worth checking the soil type. If it is clay it could be heave / shrink caused by drying or saturation changing the volume.

You can get the Hardie stuff through merchants. Beware of the thicker product Linea as it is not a stock item over here. I had to wait 4 months! All this type of cladding is surprisingly expensive. A friend used Cembrit plank and it was good value. I have a Hardie blade for the circular saw and a Bosch fibre and plaster one for the jigsaw. Non-specialist blades die quickly as the stuff acts like an abrasive. Wear a dust mask and fit the saw with a vacuum.

Not too keen on having the front door directly into the main living space. Did you see @AliG's plans for a bungalow? I thought you could do similar and incorporate the hall and study space by moving the front door.

I am surprised at how easily a lot of these membranes burn, especially bearing in mind they are in well ventilated concealed cavities.

A shattaf!

It won't hurt to put a bit of rockwool in there but if you have a 1 hour ceiling I don't think it is crucial. You can get the stuff in a plastic sleeve and you can run the battens over which holds it in place. Often you get living space above garages anyway. Fit a smoke or heat detector in the garage interlinked to the house. Actually, fit a smoke detector in every room (heat in kitchen), as the most important thing if a fire breaks out is that everyone escapes unhurt.

In reality if you have rockwool above the dividing wall compressed by the battens, fire will not travel along the battens. Make sure there is no free air space, as air is what fire likes.

If there are no external alterations I can't see why this would be a planning issue, but yes you will need to amend the building regs submission. You may as well make it completely legit as it is a one-off house in a fantastic location and no point in having it compromised should you want to sell later.

Maybe have a read through Basements For Dwellings.pdf

I listened to the first few minutes and given the subject matter I thought it would have worked better on TV. I always find describing the size of something in relation to Wembley a bit annoying too.

I assume you are doing beam and block because you are on shrinkable soil and close to trees and you were not able to do an insulated raft. If not, look at this option. Noted also that the garage floor is ground bearing and that no clayshield is specified. Have you chosen the timber frame supplier yet? Whatever, increase the ground floor insulation and allow for ufh on the ground floor. MVHR is not a big ticket item if you DIY and worth doing if you have sufficient airtightness.

Any reason you didn't just get a BFO combi?

I also read your post on white bricks, so I can see where you are with this. Just bear in mind that if you do anything other than standard it will costs £££s and will look crap if it is not done well. I am currently looking at a 1990s building that has used coloured mortar and the batches don't match, so there are bands of brick which really stick out. Have a look at https://www.brick.org.uk/archive/brick-awards-2017/ and download the pdfs for inspiration.

In the in England it is covered by Part K 5.4.

Why not continue the flat single ply roof out? You could accommodate the change of pitch and keep it as a proper warm roof. That is a very shallow pitch for tiles.

I love the look of a cut string staircase.

Door closers are no longer required for 3 storeys. Did you end up with internal drained cavity membrane, or was there a simpler solution? That was a big hole you dug, especially as the site looks fairly flat. A friend had a house with a deep basement, part of which was a swimming pool. The neighbouring house made theirs into a squash court!

It is because they are more likely to be damp, flood prone, dark, poor mains drainage for WCs etc, difficult emergency escape and badly ventilated. I don't understand why the valuers were keener on a basement than a 3rd floor unless the third floor was badly compromised with sloping ceilings and rooflights only. My experience is a basement is valued at no more than 80% maximum of upper floors and only then if it has good natural lighting and access.

We did a basement and used king piles - drill a hole every 1.8m with an auger (maybe 1.5m lower than the excavation bottom), lower in a steel h section, concrete the bottom 1.5m, as you excavate, infill between the steels with rail sleepers (which can be removed later). The more critical bits, where there were adjacent buildings, we did augered holes and filled with concrete to create a sort of secant piled wall. It meant we had a safe working space between the outside of the basement walls and the face of the excavation. Basements are a substantial risk and cost. I have experienced higher per metre build costs, while the space created is worth a fair bit less than above ground.

Not sure where you are with this but for the permanent supply it is preferable just to have a single length of pipe from the meter to the stopcock inside the house, with no connectors. Even if you have a temp supply to the caravan I would run to the house separately and reconnect. Also some people go for 32mm instead of 25mm but please don't ask me why!

It would be really helpful if you could post a picture so we can see what the current windows look like. Crittal did a lot of metal windows between 1930 and 1960, but as you are looking at Accoya I assume the current ones are timber.

This https://www.rightmove.co.uk/property-for-sale/property-76396397.html Is nearer to the size

I had a firm replace an asbestos garage roof with a double skin metal insulated roof. It was en-bloc and staggered up to the one neighbour and down to the other. They measured, removed existing, installed, provided extra bearing where needed £1,600 plus VAT with an asbestos disposal ticket. They had to move some of the crap in the garage. New roof is safe to walk on and no condensation. With the floor I just put down DPC and laid 18mm chipboard flooring over £105 plus VAT diy. New Hormann garage door £435 plus VAT diy fitted. Overall, £2,140 plus VAT for the upgrade, but now I can store everything in the dry and it is secure. Single garage

Depending on soil type, you need up to 30" (750mm) of post in the ground. Posts tend to fail and rot at the ground level junction, so when you concrete in, bring the concrete up past finished ground level and haunch up round the post, so that water runs off.

The chords look a bit thin for the 12mm fixings to get a good bite and some of them will probably hit the metal web. Because the whole of the channel would need to fail, you would notice that the glazing seemed loose before it became critical. In this location you are not likely to get a large number of people squashed up to the glass in the way you might if this was a balcony, but you need to consider the risk if, say, a future owner lets this as an AirBnB to a stag party.