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On the building regs side, if your build, especially mentioning cladding, is encroaching closer than a meter to the boundary don't be surprised if you need to prove it's non-combustible, prevents spread of fire

Maybe SSE would upgrade your existing supply to three phase at a reasonable cost (versus 21k!) and that might give you more options, like splitting into three single phases? (Bear in mind I'm a light bulb changer and plug wirer, I'm not in the trade!)

Sounds an interesting project, but do you ever intend to sell? Regardless of your engineering experience a secretly built DIY basement with no planning, building control or SE design would be a big red flag for any buyer or surveyor.

@Dunc +1 we did the same calculation - similar conductivity, but the high strength aircrete are much cheaper and easier to install

Is the rad fitted with a thermostatic radiator valve? Sometimes if they've been turned off the pin inside can stick down - you can try taking the control head off so you have see the pin position - with the head off the pin should be raised and you should be able to push it down. If it's stuck down, try (gently!) pulling with some pliers to free it.

There will usually be a condition along the lines of "The development shall be carried out strictly in accordance with the approved plans and drawings" and that allows you to make updates to the plans if you're updating the design

Getting a heat loss survey might be worthwhile, see where the heat is going. Have you weighed up internal versus external? External may initially look more expensive but internal is very disruptive, even room at a time. What would your plans be for handling kitchen and bathrooms? Refitting those will be a big added cost. Both are an (expensive) opportunity to refresh - maybe add a more modern look externally, or a good time to rewire etc internally. We've done internal but only fully renovating an old solid wall property, I'd lean towards external on an 80s bungalow - better insulation, less condensation risk, less disruption, but probably more ££ and less DIY friendly

Looks like you've a gallery landing open to the kitchen space below. Have you looked at the fire safety impact of that? Possibly fire doors on the bedrooms and windows suitable as means of escape.

Potential issues are likely to be around over-development, negative visual impact, highway safety, parking. I'd draw a line following the front line of properties left of pic and try and plan within that, which probably means building closer to your neighbour, planners would probably baulk at anything intruding closer to the corner than that. Is there any negative planning history for the site or nearby? Are you in a conservation area? There's inevitably a fair bit of risk (and cost). What are your plans and particularly what's your plan B if you can't get permission? If you're going to live in one or the other, would you be happy falling back to a large extension or terraced property that wraps around that corner?

Looks a neat job but hard to see building control being happy with slabs up almost to DPC by the render and brickwork. Check what they'll sign off before paying in full. How high is your bifold threshold and how exposed is this elevation? If you end up having to re-lay this an Aco drain would give you better protection from driving rain flooding in from your patio and seems to be a commonly specified detail. (You could experiment with a hose/bucket to hopefully reassure yourself about this)

Pretty sure the geotextile membrane's heavier duty than weed fabric, it needs to keep soil etc out for the lifetime of the soakaway. No fill (if you go for rubble instead of crates you need approx x3 the size of soakaway. What you're looking for is a silt trap before the soakaway (that site's got quite a bit more useful info too, including for a percolation test to establish size if you've not done one)

Hard to say beyond "It depends" as so much can vary. If you can keep spans small and don't have to run a soil pipe far or perpendicular to joists that will help, and be wary of the amount of deflection. I remember seeing in this thread that a 195mm easi joist may work a bit better than a 202mm posi. 225mm is generally easier though as it works with blocks.

Government permitted development advice looks like it'll work to your advantage... Where ground level is not uniform (for example if the ground is sloping), then the ground level is the highest part of the surface of the ground next to the building.) You could apply for a certificate of lawful development to remove any doubt

The first elevation looks pretty close to your image and you've not given a huge budget to evolve other ideas so you may need to temper expectations a bit or give a bit of feedback and budget to evolve the design - @ETC's sketches are a nice start in terms of ideas to build on the original. Personally I'd prefer the hall to be more central to the main rooms (and of course stairs to line up). It sounds like you're not too impressed so far though - cheap is great but not if it means a battle to direct your architect to deliver what you want. In terms of the application, perhaps show the new and old elevations overlaid so it's clear you're not actually going much if any higher at the ridge, and make another attempt to win over the neighbours, telling them how much the new design means to you and why. In person may work better than email.

Not an expert but isn't the effective depth only measured from the inner of the inlet?

Another approach to getting the sizing right is to use formers made to your planned sizes and build the brickwork to them, either timber or pre-made cavity closers (we're considering this to workaround windows with a long lead time causing delays if we wait to measure the built openings). That does mean taking on the risk for getting the opening sizings correct though.

Won't you also need some additional height for flashing where the parapet meets the main wall? Maybe look at alternatives to a parapet if it's too tight - could you create a fall from front to back so that all the water drains to the rear (see pic) or fall towards the front into a hidden gutter without a parapet. (Not an architect by the way!)

Demolition will probably be treated as commencement (google "demolition is commencement for cil" for examples) particularly if mentioned in any conditions, so tread carefully and as @Alan Ambrose says, double check with the local CIL team and get something in writing before you consider going that far.

Did they explain why? You've a long journey ahead with your architect, at this early stage you'd hope they'd be listening to your point of view in an area where you've expertise and experience. For me as a client it's important to know that an architect or builder is going to listen to what I want and, if something really doesn't work, explain why when suggesting another approach.

Velfac do top guided that height. Guided have more support for the window than top hung and are easier to clean from inside. You can see the difference in how they open on their website. (Same parent company as Rationel mentioned above)

Purely from a value perspective, even if similarly sized a new detached and the existing semi will carry quite a premium versus an end of terrace and a mid terrace (which is what the property you're buying will become). You may also find more pushback from your adjoining neighbour to the idea that their own home will be turned from a semi into an end of terrace. Perhaps get pre-app advice on whether planners prefer the detached option or the terracing. Which do you plan to live in?

Doubtful you'd be able to acquire both alley and garage at a sensible cost, and still a risk you wouldn't get planning. If you're looking for other options, maybe ask local estate agents if adding a side extension and splitting the larger property into two maisonettes might work financially. Could be a lot of work for possibly not much reward though. (Negotiating access via the alley might help here for bins etc)

I prefer your alternative, it has more of an open plan feel and adds a downstairs toilet which I'd expect in a property that size. Maybe you can tweak the design, turn the toilet around so the door feels like it opens less into the space and more into the end of the hall (could it knock through into that closet?). Maybe look at squeezing the pantry into the now quite large laundry too. Guess you've roof windows somewhere?

https://pa.midkent.gov.uk/online-applications/search.do?action=simple&searchType=BuildingControl looks like your local building control search from your other answers. Speaking to your future neighbours may get you the same info, plus a recommendation of who did their work