Advice on size of extension and floor space

hoopsbhoy

Hi,
Quick question on size of extension in terms of floor space.
I agreed with the builder that the extension was to be 11ft (L) x 14ft (W) in size. My requirement was for 11ft x 14ft of floor space and this was my interpretation of what was agreed. 154 square foot extension.
However, the builder has today poured the foundations and the outer edge of the foundation is 11ft 2in from the existing back wall. I'm assuming that the 9" cavity block will go in the middle of the foundations leaving the outside of the block approx 10'9" from the existing back wall and with the width of the block, insulation and plasterboard I'm estimating that my floorspace will only be about 9'9" x 13'6".
Have I misinterpreted this or misrepresented what I wanted or has the builder made a mistake? What would be the usual interpretation of the size of an extension?
If you ask for a 150sq ft extension should you get this amount of floor space or is this usually regarded as the size of the exterior of the extension?

Any advice appreciated.

Thanks

Poor Uncle Tom

One thread on this is enough.