hi, we have a bought a new house that needs work, it is a 1900 terrae with a single extension on the back. I have had the planning office up to see if we can build on top. The extension will be 10 ft wide by 18ft long with a pitch roof. How much do you think that this should cost ( labor only ) for the shell, i will do all the inside work myself . thanks for any help that you can get