Creating stable foundations for garden room on a plateau

[sasl]

I'd like to build a medium sized (let's say 4x3 m) room at the top of my garden where there's currently the wreck of a shed. The area is at the top of a slope. There's also a drainage ditch for the adjacent farm field on the other side so the area sits on a sort of plateau. I guess this is potentially an issue for subsidence if I build a heavy structure on a concrete slab. Is a fairly thick concrete slab (atop compacted type 1 MOT) likely going to be ok for this location or should I be looking to stabilise it more - I don't know if e.g. piles are an option for a home project or maybe a retaining wall? Or a different type of foundation altogether perhaps?

Photos of the area attached. First shows highlighted in yellow the area I want to build the room, with the sloping garden up to it. Second shows a closer view and the third shows the boundary (highlighted in yellow) backing onto a drainage ditch.

 

[noseall]

I'd like to build a medium sized (let's say 4x3 m) room at the top of my garden where there's currently the wreck of a shed. The area is at the top of a slope. There's also a drainage ditch for the adjacent farm field on the other side so the area sits on a sort of plateau. I guess this is potentially an issue for subsidence if I build a heavy structure on a concrete slab. Is a fairly thick concrete slab (atop compacted type 1 MOT) likely going to be ok for this location or should I be looking to stabilise it more - I don't know if e.g. piles are an option for a home project or maybe a retaining wall? Or a different type of foundation altogether perhaps?

Photos of the area attached. First shows highlighted in yellow the area I want to build the room, with the sloping garden up to it. Second shows a closer view and the third shows the boundary (highlighted in yellow) backing onto a drainage ditch.

The only real issue I see, apart from the effort to get rid of the volume of vegetative matter, is the tree and its roots.

 

[mrrusty]

What sort of ground is it under the topsoil? As getting a digger in causes a lot of disruption and waste to clear, why not approach one of the ground screw contractors and see if ground screws are suitable for your ground? If they say it's suitable, its an easy and clean method for a garden room. I recently used easypads, but we have solid clay only 400mm under the topsoil.

 

[Dereekoo]

The only real issue I see, apart from the effort to get rid of the volume of vegetative matter, is the tree and its roots.

+1 What ever type of foundation you use the load will spread downwards at 45 degrees which will keep it well within the confines of the adjacent bank slope and the actual bearing pressure caused by the garden room will be at the low end of any allowable ground bearing capacity. Basically you are building another shed not a house

 

[Ivor Windybottom]

If it sits on topsoil it will move, whatever miracle wonder products claim about their super slabs or whatever they call them!

Our 20 year old shed was like a fairground haunted house, all buckled and bent and the floor creaked and moved as you walked. Someone had carefully laid slabs on sand, got everything nice and level. A couple of decades of worms and moles later, it was a complete wreck.

Dig down and find something that's not black. Build up from that. Anything else is a cowboy job.

If you put a concrete slab on that lot it will crack if it's thin, or even if you build it to airport runway standards it will eventually tip.

 

[mrrusty]

If it sits on topsoil it will move

As @Ivor Windybottom says - this is the key piece of information. If you're on topsoil or made-up ground then anything sitting on the top will likely move. You need to get down to something solid. It doesn't necessarily have to be very deep, just good solid ground.

 

[sasl]

Thanks everyone! Indeed I planned to dig down to hit the clay, which in my garden at least isn't that far below the surface - at least elsewhere in the garden. The tree will be coming out most likely but I'll have to deal with the roots.

If you put a concrete slab on that lot it will crack if it's thin, or even if you build it to airport runway standards it will eventually tip.

What would you call too thin? I'm happy to make it thick, it only needs built once and best to do it right.