Price rate for increase in area when deed is rectified

I and the owner have agreed to a deed of rectification where my area increases by about 5%.  I understand that there is a stamp duty involved. 

The question I have is how much should I pay the owner, if any, in a deed of rectification where the area increases? Am I required to pay in the first place? If so, at current market rate or rate prevailing at the time of the original sale deed which happens to be about 20 years back in this case.

Any information would be helpful.  

Thanks