The project architects had hoped for achieving an 80% CO2 decrease by replacing all existing appliances with newer more efficient ones; by increasing full wall insulation; by adding underfloor insulation; and etc.
Instead, a 60% CO2 reduction was achieved. The estimated cost of doing the same for every UK home would amount to a total of $5.6 trillion, which is about $193,000 per home (there are 29 million UK homes). The payback period from the energy savings for each home owner would be 150 years.
Assuming that same style of retrofit was done for the approximately 138 million U.S. homes, which tend to be larger than UK homes, the total retrofit cost would be some $26 trillion for just a 50-60% CO2 reduction.
And that's just for the U.S. residential units. Add on the retrofit cost of all commercial, industrial, and government structures and entire cost for the U.S. would be significantly higher.
It's not just the UK retrofit project that throws real shade on the Green New Deal's insanely expensive concept of retrofitting homes for "maximum" CO2 reductions. A study out of Australia suggests that decreases in both CO2 and total home energy usage would not be significant in long run.