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.

Initial stages in planning my garden room

We live in a Wiltshire village, United Kingdom and we are in the initial stages of planning to build a garden room at the bottom of our garden in place of an existing shed. 

We have an 1890's Victorian terraced house and were looking to build an extension, however when looking at the costs involved and what we would actually gain, we decided against it as it would mean loosing valuable patio space and mean our existing kitchen would have to be re-modelled. 

What would make a difference to family life however would be the creation of a utility room plus a playroom for the children.  We have therefore decided to demolish and rebuild our shed (currently measuring 5m x 1.6m) creating a larger structure (5m x 3m) keeping one part as a shed and having a utility room in the main part of the building.  We are also looking at creating a mezzanine level which will be above the shed section and cantilever out over the utility room, this will be a playroom for the children accessed by a ladder fixed to the wall.

We are hoping that the project will fall within 'permitted development' but due to the fact we need to sink the shed partially into the ground in order to maintain it's existing roof line, and to create more head height inside, we do need to submit a 'do I need planning permission?' document to our local council which costs £30.

I have met with an architect and we are having plans drawn up currently.