Sell & Allotment of Open Car Parking

I have purchased a flat in single storied building, when the agreement was signed, I have not chosen the parking, so agreement is mentioned as with out car parking. Now I want to go for a parking space, as the covered parking area charges are high(5L), I am thinking to take an open car parking are. 1) I talked to the developer & he said that he will allot me an open parking space against 3lac rupees, as he said he will provide me a supplementary agreement where he will mention my open car parking slot & he advised me to get the notary for the same. As I came to know that open parking area belongs to the building common are & all flat owners have the equal rights to the common area, though it's not distributed or marked. If I take the parking open parking space, in future can any flat owner challange the same & at that time my supplementary agreement or notary paper can overrule the challange. 2) Developer is taking charges 40 rupees per sq ft (individually as per the respective owner flat sq ft, super built up area including common space, that is 872 sqft built up area + 25% common = 1090 sq ft) for electrical installment. This is also mentioned in the agreement, is it legal for the developer to charge for the same??