Masonry, high mass, U=0.1 or less design out thermal bridging, 3g, U= 0.7, air tight, comfortable

Velux windows can be noisy when it is raining,

Definitely demolish and rebuild - why not go for proper two storeys they do in my road

Good luck

Segal is fun but, if you are looking at timber frame, stud is easier, I think. I like chunky frames, so my recent extension was post-and-beam (not really 'Segal' as Segal Method is based on uncut materials sizes and mine is not), but if I'd flung up studwork it would have been far, far quicker!

"Can you reuse the foundations"

No VAT reclaimable if you do. Don't even use a section of the existing foundations

"Can you reuse the foundations and GF walls of your bungalow"

VAT may not be reclaimable if you reuse the walls

Timber frame all the way for me, though I'd use a small local co, not some big name like Scotframe

Novel build approaches like ICF are wonderful and all, but try explaining how your house is built to a bank or insurance co when you want a normal mortgage to take over your cripplingly expensive self build mortgage, or explain how it would be rebuilt

Also, I know it's tempting to whack clever plans in that make max use of space to squeeze a 5 bedroom house in or whatever, but consider how your house will be rated for council tax purposes when complete. You'd be better off making it as normal and small (2 bed) appearing as possible for completion and assessment then extending into the loft void (because you deliberately didn't choose trusses, and you had the framing for veluxes put in but you roofed over it etc) or cellar, garage etc afterwards - council tax is only reassessed when a property is sold. Saving X hundreds every month for the rest of your life makes sense to me

You can only make a VAT claim once, so if you run short of cash and you're DIYing, try and get the materials in even if you haven't fitted them by the time the claim comes

I stand corrected (and cannot now find the source I formerly had in mind where reuse of the old foundations had precluded a VAT reclaim)

I would say, though, weigh up carefully the costs of reusing old foundations and the limitations on your dreams they may impose versus crushing everything of the old house up for hardcore and starting over ..

Why, tony? Seems a fair bet that extg founds, even slab, could be incorporated without technically compromising even a full-PH build. Of course if it's in the wrong place on the site or similar then it'd be stupid to compromise the whole plan or layout.

This is news to me, that slab etc can be kept - thanks Will

Might be bit of a waste of time insulating to U=0.1, if you're also going to tip tonnes of new concrete into a hole, the cement emissions will dwarf the insulation savings...

Example here:
https://www.ribaj.com/intelligence/decarbonise-foundations-sustainability-climate-action-webb-yates

New conc slab + trench footings : 50 tonnes CO2 emissions
New slab + pad footings : 20 tonnes CO2
Screw piles + timber susp floor : 1.6 tonnes CO2

Using pre existing foundations which are already there : zero tonnes CO2 !

Comparison:

Insulating floor to U= 0 1 instead of to Building Regs : saves <1 tonne CO2 (lifetime direct electric heating)


Edit : an example from the same series: they took the roof off this bungalow, insulated the extg foundations and GF walls, built a new timber first floor, put the original roof tiles back on the top!
https://www.ribaj.com/intelligence/energy-efficiency-group-19-kiln-ride-bungalow-deep-retrofit

Posted By: revorNew foundations are great if done properly. 36 homes built by Barratt in Cambridgeshire to be demolished due to inadequate foundations to withstand heave. Owners of houses already occupied very concerned (despite assurances) that their homes may be similarly affected. How could they get it so wrong? £800K a pop? 3rd party BC ?

The quality of build done by the big construction companies is shockingly bad. I have seen quite a lot of new builds in my area and have yet to find one where it meets BR in the loft. Missing or incorrectly spaced trusses though to no bracing. None of them met airtightness regs.

Building control is done on one property and if that passes it is 'assumed all the others will too. Its a shocking situation and I feel sorry for the hundreds of thousands of people who have bought over priced, sub standard houses in the last couple of decades.

We went with CLT for our 1.5 bungalow but the insulation depth effectively forced us to overclad the outside with engineered timber joists (Steico) in a Larsen Truss arrangement as hangers weren't deep enough.

If I was building again on a smaller budget (carbon and financial), I'd definitely consider just building with the Steico joists as they were relatively cheap and very strong. There's a compatible wood fibre insulation system where they fill the joist webs to get smooth sides and then you can insulate between joists with either more wood fibre or blown cellulose.

https://www.steico.com/en/solutions/new-construction/wall-construction

As others have said, you just need to get the timber up off the ground then so it stays dry.

If you're looking for companies to do it for you, then it's worth looking at Beattie Passive who offer a full design & build system to certified Passivhaus which works on a similar basis:
https://www.beattiepassive.com/our-system/technical/

Like electrochemical steel, and e-petrol, the premise is that it will be made once copious spare renewable electricity becomes available all around the world (in the next decade or three... probably from all the fusion reactors the same kinds of companies 'have a plan' for by then!)

But long before then, we will heat our timber homes with a little of that copious spare renewable electricity, and the carbon savings from the insulation will become nill!

So I don't think I spoke too soon - no need to delay building your house till 2050 :-)

Edit: and of course the electrolysis still releases the CO2 from limestone to make lime, they are relying on copious spare CCS capacity becoming available to deal with that. And the lime needs to be sintered together with the silcates at 1400⁰ to make cement, that will need copious spare green hydrogen fuel. Better allow another decade or two for those bits...!

Well, yes! Dr Ellis is upfront in her publications that

"we liberate the CO2 as a gaseous product to be captured and sequestered", and

"mix (electrolytic lime) with SiO2 to form [cement, by] heating for 2 h at 1,500 °C"

I did include the smiley! :-)

https://www.pnas.org/doi/10.1073/pnas.1821673116

Actually, it's great that people are thinking about this stuff. Moore's Law says that anything new coming out of labs as of now is not going to scale up to gigatons in time to be the climate silver bullet, but might still be handy later.

Couldn't find it before but I came across Baufritz recently. https://www.baufritz.com/uk

Recent announcement of an improvement to glass that should cut its carbon emissions. https://www.psu.edu/news/research/story/new-glass-cuts-carbon-footprint-nearly-half-and-10x-more-damage-resistant/

Another one where they claim to have reduced the process temperatures. "LionGlass eliminates the use of carbon-containing batch materials and significantly lowers the melting temperature of glass.” And they also seem to be stating they are using something other than limestone and soda ash. So there is some good news around.

WillInAberdeen said: "Better to hold off on any new buildings, until they can be glazed with transparent wood :-)"
https://phys.org/news/2021-05-citrus-derivative-transparent-wood-percent.html

Hmm, seems like it's difficult to buy; https://inventwood.com/transparent-wood/ is as close as I've found.
But if you're up for a bit of DIY you can make your own: https://www.instructables.com/Transparent-Wood/ :)

BTW, the LionGlass looks like just what the Wine Society are looking for to complete their net-zero plan.