Meeting with local builder booked

I have a meeting booked with a local builder for Sat 8th November. By then I should have paper plans of our build from our architect.  The whole process seems to be so slow - but I am just impatient!

I have a whole list of questions to ask the builder - mainly about construction materials and keeping out the damp.  Due to the limited space we have available, we need to have walls as thin as possible but we do need the building to be thermally efficient.  It will be used on a regular basis so does need to be warmer than a normal garden shed.  I would like the building to look like an old brick outhouse which has always been there - using reclaimed brick on the front and having a sloping slate roof. 

We cannot build higher than 2.5m as it will up against a boundary wall, we are further limited in the height of the building as we want to keep the maximum roof height to below the boundary wall at the end of our garden - we are in a conservation area so want to make the building to be as low impact as possible.  We are therefore needing to dig the shed into the ground by approx 1.5m in order to have a reasonable head height inside the building.  I am also determined to add a low mezzanine level too which will act as useful eaves storage and an area for the kids to use.

My main concerns at the moment is finding the right construction material to make the building as big as possible inside - 5 x 3m is not very big so we cannot have thick walls.  I also have concerns about making the structure water and damp proof considering we are digging it into the ground.  I am hoping a good meeting with a local builder will answer most of my questions.