Much of what we need to do to live more sustainably costs very little: LED light bulbs are coming down in price all the time and changing our lifestyle might costs nothing. However, insulation (see Energy) and air conditioning (see Health) done seriously are a different matter and most flat-dwellers will need to look for external funding.
I have not had any success in finding such, but society is still in turmoil with Covid-19, and there is a possibility of a green renaissance instead of the same Carbon-burning catastrophe. I’ll keep you updated.