Charge Maintenance For Private Terrace

We are a Housing Society registered under Karnataka Society Registration Act 1960. One of our members has procured from the builder an area (approx 950 sq ft) between two flats at the ground level. The access to this is from his flat alone. This is mentioned in his sale deed as "open terraces area". His undivided share, super built up area is same as others in his block. We as society were not aware of this and since inception in 2017 have been charging him the same monthly maintenance charge as others with his sqft area. Now when we came to know of this, we were of the opinion that since he has more area, he should pay more, however he contests the same citing Property Tax calculations where Open Terraces are not charged for. Our bye laws talk about charges for maintenance but does not cover how it should be calculated. In our AGM we had decided to go as per sqft but did not mention explicitly what we should use, however we have been using super built up area. Can someone please guide