1. you need to check the revised NGT circular whether it has a prospective or retrospective application
2. it may be that pursuant to the earlier circular, the local body may have sanctioned building plans complying with that circular and the builder must have commenced construction on basis of such sanction
3. so if the construction has already begun and subsequently a new circular imposing a higher limit for maintaining distance from a secondary nalla is issued, then such a circular, in my opinion, would be applicable on prospective basis. Meaning all applications for sanction of building plans received after the date of the revised circular would have to comply with that circular. This also means that all sanctions given prior to this revised circular would remain unaffected if construction has already commenced on site
4. so please check the wording of the revised circular to ascertain its applicability to the project in which you wish to purchase the flat
5. you can very well approach banks to finance the purchase of the flat you intend to buy
6.. if due to some reasons the local body does not issue OC for the building, then your possession will be stuck and you may find it difficult to sell this flat