Demolition and rebuild really isn't a good idea, however much you make the building better insulated etc etc etc.

The data says:

 o    BRE and also the Royal Institute of Chartered Surveyors (RICS) in their Whole Life Carbon Assessment for the Build Environment (2017) rate demolition of medium density urban structures in the range of 10–20 kg CO₂ per square metre

 o    The UK Green Building Council (UKGBC) estimates that embodied Carbon for New Builds for mixed-use including high rise the figure is 800-1,000 kg CO₂ per square metre while RICS gives an even wider range of 500-1,000 kg CO₂ per square metre, the more concrete the higher the figure.

 o   Embodied carbon for reusing and refurbishing existing buildings according to RICS  (2017) would require 50–70% less embodied carbon compared to new construction  while the UKGBC Net Zero Carbon Buildings Framework (2019) suggests a figure of 300–500 kg CO₂ per square metre.

 So re-use is about a third the carbon cost of rebuilding.

Those numbers look quite low but I guess I'm thinking of the Holloway redevelopment site with skyscrapers.

Do those estimated ranges only apply to the "medium density urban structures" you quote? I suppose I mean to ask what that means.

Medium density falls between suburban - typically two storey, often semi-detached - and high rise blocks. Currently it would be anything from five to eight storeys.

Typically, as soon as you go over four storeys you triple build cost because it involves a lot more steel and concrete - bigger foundations for example, and more framework to defend against increased wind pressure. So yes, the taller you go, the bigger the carbon footprint - and the bigger the footprint of running it because it's a rare 17th storey resident who will walk up and down rather than using the lift. And even water requires power to pump it up to all the residents.

The densest layout is actually four to five-storey street facing property (think traditional housing in eg Kensington & Chelsea) which can do disabled access with stair lifts and living space with decent light levels. (Towers when they are built close together are very dark for residents on lower floors.)

    Kate Calvert The densest layout is actually four to five-storey street facing property (think traditional housing in eg Kensington & Chelsea) which can do disabled access with stair lifts and living space with decent light levels. (Towers when they are built close together are very dark for residents on lower floors.)

    But how can four to five storeys be denser than 20 storeys? I assume you mean density of inhabitants?

      Densest population - in decent housing. Slums - new build or not - are a different matter.

      Sorry I am the one being dense now probably, more than the housing. How can 4 or 5 storeys provide denser housing when you can build it more storeys on top and make it denser?

      The taller blocks take up space on each floor for lifts, lift lobbies, stairs, fire stairs, nowadays a second lift for up to eight members of the fire services - and all those extra thick walls mentioned above. They also have to allow breathing space around each block.

      So yes, you could build like an ants nest and create very small units with limited light, but that is not housing you would want to live in. See the image at https://pbs.twimg.com/media/DzzclQGUUAAkwZt.jpg for comparisons.

      OK, that makes sense although there's obviously a grey area there around what is acceptable and what is an ant's nest, I suspect, that will give a bit of leeway to build a few more units on the top. Reminds me of the government's retraction yesterday of regulations to make new builds beautiful. Presumably there wasn't an objective definition of what that meant?

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