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No way an equivalent new build would be cheaper. Also it would be an embodied carbon and resource extraction disaster. We need to make it easier to retrofit, not demolish.
hoefla-
It's pretty easy to retrofit, just not aesthetically pleasing. A guy on my road has slapped 20cm of insulation on the outside of his and rendered over it. Looks like a brand new house. Do that all the way around and sort the roof insulation and you're golden. The house just looks like a dull rectangle with none of the original features. And is still crumbly and crap on the inside!
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If you're talking about fully gutting and retrofitting a bog standard 3-bed Victorian terrace to 2023 new-build regulations using common construction techniques and materials (requiring petrochemical-based insulation on all external walls and suspended floors, new windows & doors, extensive airtightness strategy, chimney stack work etc etc), and extending the loft and ground floor (requiring shit-tons of concrete and steelwork)...
...the total cost will be higher, and embodied carbon neither here nor there, compared to a modern timber-framed replacement of equivalent total volume, with cellulose/wood fibre insulation, timber-framed glazing, efficient infrastructure (MVHR etc).
Sparky
ectoplasmosis
I lament the inflexible planning laws in the UK that cockblock you being able to flatten your busted old Victorian dump and build a near-passive replacement for far less cost than extending and retrofitting.
Being in a modern, well-insulated, well-constructed building with good airtightness and ventilation is an absolute revelation compared to the bulk of UK housing stock...