Sunken shed

by Toby
(Fareham, Hants, UK)

Hi all. I'd like to erect a 12'x12' substantial shed to use as an art space. It will be near a boundary and the regs state that it can only be a max of 2500mm high as a consequence. However, I still want a decent internal height so am trying to work out how to "sink" the internal floor area below ground level to maintain the internal height.

I plan on laying a thick concrete base which is larger than the shed footprint, say 14'x'14' so that the concrete edges can help withstand the side forces from surrounding soil.

Then I plan to use blockwork (laid on their sides) to produce a sturdy wall which reaches above ground level and a shortened shed will sit on top of this wall. This will be on 3 sides but for the front (ie door) side the shed will be full height.

The interior walls, ceiling and floor will have probably 4" of rigid foam (Celotex, Kingspan etc.) insulation and all doors and windows will be double glazed.

The challenge is waterproofing the base and lower walls. I thought about perhaps a bitumen paint over the concrete walls and blocks, overlaid with a heavy duty polythene damp proof layer. The floor and wall insulation would then be on the interior of the insulation and stay dry.

Any thoughts about problems I might have missed or any suggestions for other options?

many thanks

Toby