in renting of flats sq. ft area in carpet or builtup does not matter.
its very clear - this is flat and rent is so much.
(for eg. your flat is 1000 sft carpet and 1250 sft built up and in that area normal rent for such flat is 15000 per month inclusive of society / building maintenance and corporation tax then then it 15000)
it does not matter whether its on builtup area or carpet area.
secondly your tenant is not paying rent for 20 years and you hv not taken any legal action against him for eviction so his adverse possession is proved and now eviction is very difficult.
if he is ready to pay you rent willingly then take it at least you are getting some thing and he is accepting that he is still tenant of yours.
thanks,
Pande Advocates & Legal Consultants
pune - 9823054740