Since it is part of agreement for sale of flat, it will be composite supply and liable to tax as per tax of Principal Supply. Since the Principal supply is taxable at 1% or 5%, the development charges and infrastructure charges will also be liable to 1% or 5% as the case may be.
However reference can be made to some contradictory advance rulings. In case of M/s Puranik Builders Ltd. (GST-ARA-68/2019-20/B-52 dated 27.08.2021) based on fact that for stamp duty purpose, other charges are not considered as not part of sale price of flat, the Maharashtra AAR has ruled that it is separate service liable to tax separately at 18%.
In advance ruling in case of Bengal Peerless Housing Development Company Ltd. 2019-VIL-130-AAR dated 02.05.2019 the WB AAR has held that charges towards common facilities are part of Composite supply of construction service of flat and therefore liable at the same rate applicable to sale of flat.
Thus, there are different views but in my opinion the development charges and other charges mentioned in agreement are composite supply and liable at applicable rate of 1% or 5%.