Construction Area Calculation For Complete Contract - Need help

[karthik.nitw]

Dear members,

Before we started house construction, contract agreement drafted by my engineer cum architect has mentioned total construction area (in sft) based on floor plan i.e concrete slab area. And per sft rate and total construction cost are written clearly.
Now at the finishing stage, after the outside walls plastering & one coat of primer are done, my contractor is measuring roof area from parapet wall end to end. He is showing the built area difference almost 20 sft more per floor. He says that this is the common way of measuring the roof area by any contractor.
Kindly help me to understand few things here:
1) How is total construction area is calculated in material contract? Just concrete slab area as we mentioned in our agreement or is the roof area calculated after plasterwork?
2) When we have already signed agreement, what's the point of recalculation? 1-1.5" thick plastered walls on each side would make a big difference in area calculation.
3) If we want to reduce plastering width on each side, how much thickness should I consider?

Your valuable inputs are very much appreciated.

Thanks in advance!
Karthik

[navhe]

Usually contractors do this. Difficult to argue. just 20sqft increase/floor does not seem to be much of a difference.
if he has done satisfactory job till now, you may agree to that measurement and make sure he would not increase it further.!!

in my case, it was increased by more than 200sqft and it was a shocker for me!! Contractor started counting extension portion of the slab also which was acting as chajja. I had increased headroom slab portion to make it common for solar/water-tank as well. Measurement was done End-2-end for Outer-wall with plastering finish. So, 3 inch (1.5 inch on each side) overall increase is unavoidable.