Measure it from the outside walls instead of from the inside walls.
I am not trying to be facetious but I just want to warn you that there are many unscrupulous people who will do exactly what I suggested to get a few more sq ft. There are some who will also round off everything upwards. They will measure something that is 12.5 x 14.1 and they'll call it 13 x 15. That is obviously plain stupid and such people are obviously asking for problems.
If your question is about cost effectiveness, then I suggest that you contact a few builders and ask the whether it is cheaper to build up or sideways.
My 2 cents of opinion right there is that you should always be careful before expanding a home. What you risk doing is making your home the most expensive house on the street. This is bad for resale. You should always be the cheapest one.