As a previous posting has said, "no break-down of the 'essential priority' works has been made publicly available" and so is not possible to independently verify the figures produced by the District Council.
Futures Forum: Knowle: old bricks vs new build: embodied carbon
Therefore, any attempt at calculating the 'embodied carbon' of any proposed new-build for the District Council's HQ is very difficult. Despite a published figure of
a new purpose built office of some 4,225m2 built to a BREEAM Very Good standard
no specifications of this 'purpose-built office' have been made available to the public.
Here are three attempts to arrive at such a figure, using the on-line 'carbon assessment tool' from eco2.phlorum.com:
The most environmentally-friendly construction would produce CO2 emissions of 1,100 tonnes - for a relatively smaller build in terms of building perimeter.
For a slightly larger build (but keeping the overall square meters) the impact is larger - at 2,300 tonnes of embodied carbon.
This has the highest impact - at 4,200 tonnes of emissions for a new-build - but the boxes ticked were average or below-average, whilst the District Council refers to BREEAM standards.
The impact could vary considerably - but there would still be an impact running to well over a thousand tonnes of carbon produced in building a brand new set of offices.