I'm consider building the classic end of the garden utility building
3m/8m
Single story mono pitch PV roof, with work store, bike shed and office/sitting around space.

First thought was stick build with OSB , featheredge , insulated office space only
from the inside : OSB ,quilt in between stud, some board insulation outside studs,baton, felt, baton, featheredge.

Then I thought of 100mm block, with pillars , 150-200mm EPS , thincoat render.
which is simple and probably cheaper.
Be good to not need to heat it.

then I thought of digging a cellar !

It's just a case of digging a hole, slab at the bottom and another 2-2.4m of blockwork, perhaps 150-225mm
and sticking a floor in, right ?
So I'd drop the EWI about 1m below gnd level and tank it up to DPC internally.

Any bright ideas on how to do it better, especially the cellar bit ?

I was thinking full basement Tom due to permit development height restrictions on garden buildings being 3m to eaves I think.
But yes semi basements are a great idea shame it's not done more in the UK.
I was just wondering what the best environmentally conscious solution to such a building would be. ( other than not building it at all)

Posted By: jamesingram3m to eaves

How about low-eave room-in-roof?

I think the hight should be measured from ground level - if the site slopes from the highest ground level next to the new building. This link (https://planningjungle.com/wp-content/uploads/Part-1-of-the-GPDO-The-10-Worst-Permitted-Development-Loopholes.pdf) shows what is possible for height at No.2

The document may be out of date - don't take it as gospel.

Once you go to more than 1 floor then building regs come into play.

In my case I was trying to get them to agree that the (higher) decking level was the 'highest ground level', whereas they took the view that it was simply the highest *approach level*, and that I was entering 1m above ground level.

Rust?

Posted By: goodevansMy take is that both are wrong. Heights should taken from the highest ground level around the proposed building perimeter without cheating by filling, excavating or decking.

It's easy to imagine why authorities would take the absolute opposite view, i.e. taking height form the lowest ground level, so that the maximum visual impact of the building is considered?

GDPO 2015 (Consiolidated) as of June 2018 says:

"Unless the context otherwise requires, any reference in this Order to the height of a building or of plant or machinery is to be construed as a reference to its height when measured from ground level; and for the purposes of this paragraph “ground level” means the level of the surface of the ground immediately adjacent to the building or plant or machinery in question or, where the level of the surface of the ground on which it is situated or is to be situated is not uniform, the level of the highest part of the surface of the ground adjacent to it."

They can't argue with that.

I thought external tanking was more reliable than internal?

would external tanking be warmer than internal tanking?

The thought being that with external tanking the wall will be dry whilst tanking internally will give a wall wet up to the tanking and I would have thought a wet wall will conduct heat better than a dry wall.

Also tanking externally the water pressure is pushing the tanking on to the wall whereas internally the water pressure is pushing the tanking off. - does it work like that ??

Irrespective of the tanking what about EPS EWI on the cellar walls and floor - depending upon what the cellar is to be used for. And don't forget provision for cross ventilation - a couple of 110mm drain pipes as tubes up to above ground level on opposite walls should do.