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.

 

[mrrusty]

Indeed I planned to dig down to hit the clay, which in my garden at least isn't that far below the surface

If you can get down to a good clay layer easily then you should be good to go. (Our house on corbelled brick footings stands directly on a clay layer just 450mm below the surface, and has done for 120 years). I used the same clay layer as the supporting layer for my own garden room using easypads. https://easypads.co.uk/ For mine there are 12 x 2.5T pads - 2.5T means the design max load on good ground. If you give the guys a ring there or email them with your plans and ground conditions they'll recommend what you need. Dig down to the clay, fill the holes with wacked MOT to bring it up to the level you want, place easy pads, adjust for level, build wooden subframe, and build timber frame building on top of that.

No good for a brick/block building of course. My build thread is here https://community.screwfix.com/threads/garden-room.256842/ Mine has been up about 18 months now, and the skimmed walls inside don't have even the smallest movement crack, so all looking good that it's all solid. An advantage of easypads, is should one of the pads sink, you can always get a spanner on the adjuster and push it back up - not something you want to plan to do though!

I've also built a garage recently on a reinforced concrete slab sitting directly on to the ground. That hasn't moved either, but the mess/disruption of all the digging out was something I wanted to avoid for the garden room.

If I wanted to build in brick/block for a garden room, I think I'd use traditional strip foundation with a non-structural slab. Less work and cheaper than a raft.

 

[Ivor Windybottom]

I planned to build a shed last year. After reading the permitted development rules, I was surprised to read that you don't need planning permission to build in brick, in fact you can build bigger as it's fire-resistant. Also the size is based on floor area, not external size. So thicker walls don't affect how big you can build internally. Which is why I'm now building what's basically a half-sized newbuild brick house with cavity walls.

We're planning to live here for many decades, I'll be very glad to not be out with my tin of creosote when I'm 80 or looking at a collapsing rotten old shed.

So decide what you want to build first, then look at suitable fondations.

 

[mrrusty]

After reading the permitted development rules, I was surprised to read that you don't need planning permission to build in brick

@Ivor Windybottom - That's not quite correct. The materials have nothing to do with permitted development or planning.

For PD/Planning - less than 2m from boundary to be PD has to be under 2.5m high*
For BR - under 15sqm internal do whatever you like build in timber right on the boundary if you like. greater than 15sqm internal, but under 30sqm if less than 1m from boundary, has to be substantially non-combustible materials to avoid regs. Under 30sqm internal and more than 1m from boundary, no regs**, do what you like. Over 30sqm BR apply

* assuming you have PD rights - not in conservation area/listed/PD rights withdrawn
** excluding things like leccy and drainage which require regs in their own right

 

[Ivor Windybottom]

Exactly. Many people incorrectly assume (as I did) that a brick building would need planning permission. But you don't, in fact you can build bigger. Which to me makes it a non-brainer - rat infested rot box or a proper building, for similar money?!

Mine's about 18sqm, under 2.5m high (after some careful design) and just over 1m from the boundary. Would have to be smaller if timber. But it's a lot bigger externally, as it has a double brick wall with 100mm cavity. Should keep it dry, frost-free in the winter and not stupidly hot in the summer.

My long-term plan is to build it with a cheap flat roof, then later apply for planning permission to build a garage onto it with a replacement pitched roof over the whole lot. IMO you're more likely to get permission when half the building's already there. It's not a conservation area but very visible.

Brickwork can be DIYed, blockwork definitely can, both need experience e.g. of building garden walls first. If you're less confident you could build the foundations then get a brickie in, then you do the roof, windows etc.

So... my suggestion is to think before you start, and consider building a proper building.

 

[mrrusty]

and consider building a proper building

I've no particular dog in this race - the garage I built is brick and block, the garden room SIPs. Both are perfectly viable "proper building" constructions. I wouldn't discount SIPs for a garden room - easy, fast, well insulated, and exactly the same technology as a huge number of modern houses are built from. Our SIP building (with 100mm XPS insulation) requires absolutely minimal energy to heat it - it has a SIP roof as well, and 50mm PIR in the floor.