The querist is talking of applicability of GST rate @12% or 18% to single residential unit in the scenario explained by him. I am also of the same view as explained by Sh.Himansu Sekhar Ji.
It is a barter agreement between the two persons. There is no difference between 'cash' and 'kind' regarding the receipt of consideration. When 100 flats are constructed by any construction company, neither lower rate nor exemption is available to single(individual) residential flat except the various options given in F.No. 354/32/2019-TRU dated 07.05.19 affordable/non-affordable etc. The same analogy is applicable here. Single individual residential flat (unit) cannot be segregated for the purpose of benefit of lower rate of GST.It is a business and will be taxed cumulatively. The element of 'individuality' (individual/single unit) is lost/absorbed in the construction of more than one flats.