Foundations for my outbuilding

[BasicBuilder]

Hi all, I hope you are well. I'll try to keep this short.

I am building an outbuilding (hopefully to use as an office / gym). The size is approx. 7m by 4m.
It is being built on the boundary so within 1m.

I plan to build it all of the light breeze blocks (except two rows of bricks at the bottom, for DPC I believe).

My question is, how deep / wide should my foundations be?
For width: planned to do these at 400mm (100m for 4" brick / block and 150mm each side remaining).
For depth: planned to do 300mm.
Then fill with 2" of hardcore and then rest with 1/3/5 (cement, sand and ballast).

SPANNER in the works is that the neighbour on the back side of the shed has a big tree in his boundary. The roots of this are partially in my garden. Beneath where the shed will sit. I planned to do a depth of 600mm along this back wall to be safe.

Any help would be much appreciated.

 

[Scarlet Pimpernel]

I'm no expert, just a DIYer, but as no-one else has answered, I'll do my best.
For depth, you need to go down to undisturbed ground and to ground that has nothing left to compost, e.g. clay, gravel, rock etc. In other words down past any topsoil, and at least 500mm deep to bottom of trench. But the ground conditions may determine deeper.
I believe 300 mm wide is sufficient, but the ground conditions may determine different.
Here's an excellent article:
https://www.designingbuildings.co.uk/wiki/Strip_foundation

You don't use hardcore in strip foundations.

If you're doing minimum strip foundations, or trench fill foundations. you need enough brick courses to get you out of the ground, e.g. up to dpc level. although blocks can be used, it depends on your blocks, if they're OK to use below ground.

To assess the tree roots problem, you may need to partially dig the trench to assess the situation and then seek expert advice. Maybe photographs, and a question on this forum.

 

[BasicBuilder]

Thanks for responding.

I think this would be classed as trench fill foundations and I was advised to just fill it with mixtures of ballast, cement and building sand. Then do my a couple layers of bricks with DPC.
Would this not be the best way then?

 

[Scarlet Pimpernel]

When you say 300mm depth, are you referring to the thickness of the foundation, or the depth of the trench?
Otherwise, your suggested mix of concrete is OK.
And yes, brick courses to get you out of the ground and up to dpc. The bricks do not provide the dpc, you need a dpc material.
https://www.screwfix.com/p/capital-...p-proof-course-1968ga-black-30m-x-100mm/42073

 

[BasicBuilder]

300mm depth as in the depth of the trench. Then maybe 50mm hardcore and 250mm of concrete as per the above mix.

So I'd bring it to ground level and then put a layer of bricks with the DPC material above.

 

[Scarlet Pimpernel]

300mm depth as in the depth of the trench. Then maybe 50mm hardcore and 250mm of concrete as per the above mix.

So I'd bring it to ground level and then put a layer of bricks with the DPC material above.

I'd advise 450mm deep, as long as the ground is good.
No hardcore required.