7x4 garden room

[Jadhacha]

Hello, completely new to any kind of building work, so pls bear with me. 
 

So long term goal is to build an extension - but will need planning, structural engineer, and of course funds.

 

Short term, trying to build a garden room. Similar construction methods, but hopefully at a lower budget - and learn. It’s 7x4m, occasional use office/ gym/ entertainment area, no toilet/ sleeping arrangements. Ideally would like to build cavity walls out of engineering bricks till DPM, then solid blocks and flat roof.
 

Previous reading and other sources suggest 60x60cm trench foundation should be sufficient for a ‘light’ structure as garden room, the more I dig, the more I read, the more I worry. 

Few details,

* 7x4m building

* 60x60cm trench foundation 

* Soil and clay

* Tall trees nearby 

* In Pinner, Harrow 

I’ve not consulted a structural engineer, as they’re likely to suggest dig foundations to 2.5m. Have 2 friends nearby, who’ve been advised this for extension and garden room with toilet. I’ve been digging on and off for a week or so, and I’ve got to about 60x60cm. Ground is sloping, hard on one end, and clay on other, digging is difficult and with no end in sight, even more so. 
 

Do I keep digging - how much is enough? Any alternatives to make foundations as best I possibly can? Happy to be reasonable for hand digging. 

 

 

 

[Nickfromwales]

Hi. I would speak to a screw pile foundation provider, to see if this would be cheaper and simpler/quicker.

 

If just an outbuilding, then you may be able to do away with needing any concrete at all.

[Jadhacha]

Thank you for your response. I see what you’re saying. But I’m hoping to eventually use the experience to build an extension, so trying to stick to similar construction methods as residential extension. 
 

 

…and when I’m not working I’m digging, 60cm deep now. 

[Nickfromwales]

FYI, my 6m x 3.6m garden shed/office/mancave is sitting on 100mm x 100mm posts sat on top of paving slabs. In a few years, the frame has moved about 6mm, and I have had to shave the bottom of the entrance door where it began to scuff the threshold. Other than that, rock solid.

 

If I had gone for screw piles, it wouldn't have moved the 6mm, but I always knew this was a semi-temporary structure so didn't want to waste money. 

[Nickfromwales]

Just now, Jadhacha said:

Thank you for your response. I see what you’re saying. But I’m hoping to eventually use the experience to build an extension, so trying to stick to similar construction methods as residential extension. 
 

 

…and when I’m not working I’m digging, 60cm deep now. 

Then you need to use the brain and the calculator.

 

DO NOT piss money away making the outbuilding way more than it needs to be, and also (please) listen and learn here.  

 

The extension can be done the same way!!!!

[Jadhacha]

Perhaps I need a step back and look at options to build an extension and work backwards - I will pause and look into this further. Wonder why aren’t screw piles as common, I’ve seen loads of them online, but rarely any done nearby. All seem traditional concrete foundations. 
 

Thank you. 

[saveasteading]

@Jadhacha what Are they charging for these screws? 

I agree with @Nickfromwales unless these are £20 each.

I've had a chat with a screw supplier at a construction show. . They didn't appear to know anything so I think were franchisees.

[Jadhacha]

No idea. I’ve heard cost wise they’d be similar to concrete, but may save on digging, quicker and such. 

[MortarThePoint]

Consider a raft Foundation:

 

https://forum.buildhub.org.uk/topic/12840-outbuilding-foundations-raft/

 

What you've dug already could be a large ring beam

Edited yesterday at 04:57 by MortarThePoint

[MikeSharp01]

19 minutes ago, MortarThePoint said:

onsider a raft Foundation:

 

https://forum.buildhub.org.uk/topic/12840-outbuilding-foundations-raft/

 

What you've dug already could be a large ring beam

+1 to that. We built our garden room as a model for the house as timber frame using the same techniques - I Joist etc, but with thinner insulation.  However if you are going brick / block for the extension then perhaps raft is not such a good choice.

[saveasteading]

7 hours ago, Jadhacha said:

Soil and clay

* Tall trees nearby

That was one of the questions I put. They knew nothing.

 

Such screws are a possibility for a garden room, especially if you want it raised. Also for a temporary building eg a beachfront shack.

 

For an extension it must perform the same way as the house. It needs building regulations, thus an SE.

 

Tell us more. What type of tree and the distance? Is the ground clay or what?

 

It would be interesting though to get more info from these screw suppliers.

[Jadhacha]

Well, there are plenty of medium/ large trees around in 10m+ vicinity, but closest and largest is Horse Chestnut about 5m away. 
 

Options so far,

1. Keep digging (and stop short of China)

2. Raft foundation (needs homework)

3. Ground screws (I’m on the fence for this one)

4. Dig the trench in figure 8, rather than 0 (sounds made up, but might work?)

 

I’m out all day, so I can pay the mortgage and feed the family, but shall be in the trenches by 6-ish. Will post pictures then. 

[Russell griffiths]

I’m from Ruislip and worked around Pinner a lot. 
your extension will not be built on a trench 600mm deep, it will likely need a strip foundation at least 1.5m deep with mass fill concrete to within 450mm of ground level. 
I would bet you actually end up doing the extension on a percussion driven pile, this is very common around that area due to the soil and trees. 
the last extension I did in Ruislip we went down 2.3m with a strip foundation as piles were not common back then, and expensive. 
it’s far more common to use this method now. 
 

are you being misled when they say 600mm foundation, this normally means the amount of concrete, not the depth in the ground the concrete is placed at, so you actually have a 1m deep trench with 600mm of concrete in it. 
 

minimum dig depth for anything looking at needing building regs is a metre. 

[Nickfromwales]

Percussion piles indeed, not sure why I put screw piles in my text, I meant piled founds in general.

 

We’re looking at an extension project for a client in Ruislip, and the geotechnical survey shows exactly the same as above, minimum 1.2-1.5m deep to pour founds, so one grounds company now re-quoting with piled founds; to save costs we’re looking to avoid the excavation and muck away.  Have switched to looking at TimberFrame internal and brick external vs block & brick to reduce weight too. Just slow getting people to quote…..

[kandgmitchell]

Horse chestnut 5m away.... I'd design for piles from day one as I don't think digging to China is far enough from Pinner

[Jadhacha]

2 hours ago, kandgmitchell said:

Horse chestnut 5m away.... I'd design for piles from day one as I don't think digging to China is far enough from Pinner

I was worried someone’s going to say this!

[Jadhacha]

4 hours ago, Russell griffiths said:

I’m from Ruislip and worked around Pinner a lot. 
your extension will not be built on a trench 600mm deep, it will likely need a strip foundation at least 1.5m deep with mass fill concrete to within 450mm of ground level. 
I would bet you actually end up doing the extension on a percussion driven pile, this is very common around that area due to the soil and trees. 
the last extension I did in Ruislip we went down 2.3m with a strip foundation as piles were not common back then, and expensive. 
it’s far more common to use this method now. 
 

are you being misled when they say 600mm foundation, this normally means the amount of concrete, not the depth in the ground the concrete is placed at, so you actually have a 1m deep trench with 600mm of concrete in it. 
 

minimum dig depth for anything looking at needing building regs is a metre. 

Thank you. This is very helpful, and bit worrying. 
 

Is this necessary for a garden room though?
 

There’s no building regs involved. I was hoping to dig 600 and pour 500-600 of concrete in it. I’d like to try and understand each step of ‘conventional’ building work, but not get too carried away, and blow the non existent budget!

 

If it a no go, still not too late to fill the trenches back up with MOT1 and get a timber outbuilding on it - if that’s more doable. 

[saveasteading]

17 minutes ago, Jadhacha said:

necessary for a garden room 

Yes as it is part of the house. Make it separate and it isn't.

 

 

18 minutes ago, Jadhacha said:

There’s no building regs involved.

Says who? Or do you mean you haven't contacted them?

Perhaps make it an outbuilding with a passageway link. I think I'd regard that as not affecting the house.

 

[Jadhacha]

Not sure I understand? The garden room is 35m away from the original house. Under 30sq.m, and 2m away from boundaries + no sleeping arrangements, toilet, occasional use - there’s no need for building regs that apply.

 

9 minutes ago, saveasteading said:

Yes as it is part of the house. Make it separate and it isn't.

 

 

Says who? Or do you mean you haven't contacted them?

Perhaps make it an outbuilding with a passageway link. I think I'd regard that as not affecting the house.

 

 

[saveasteading]

1 hour ago, Jadhacha said:

garden room is 35m away from the original house. Under 30sq

Begging your pardon. I'd misunderstood your intro. I now see that the extension is for another time.

This is a small building in the garden.

 

SO: the tree will affect it. The ground will shrink in summer and expand in winter by 10mm or so.

Does that matter? I'd say you live with that and any minor damage it might cause.

 

So you can use 1m deep footings and not much damage should occur.

I'd say that is OTT for this building.

 

Or slab with 400mm downstands and accept a bit more risk.

so go with your 600 depth, and 400 wide if you can fit in it.

 

To minimise ground shrinkage make sure the rainwater off the roof is dispersed very adjacent so the tree gets it.

 

 

 

[Jadhacha]

14 hours ago, MortarThePoint said:

Consider a raft Foundation:

 

https://forum.buildhub.org.uk/topic/12840-outbuilding-foundations-raft/

 

What you've dug already could be a large ring beam

Looks like I’m leaning more towards this.. more homework for tonight. 

 

Thank you @MortarThePoint

[Russell griffiths]

If you have a non existent budget then why build the garden room like an extension, build it like a garden room and save the £5-6 grand to do the extension. 
 

if you have no prior building knowledge then your not going to build the extension yourself anyway, so pointless trying to learn something. 
you would be better off learning the methods of construction so you can employ trades as and when you need them. 
 

you will need a fair chunk of money for the extension, don’t for a minute think it will be cheap. 

[Jadhacha]

Just now, Russell griffiths said:

If you have a non existent budget then why build the garden room like an extension, build it like a garden room and save the £5-6 grand to do the extension. 
 

if you have no prior building knowledge then your not going to build the extension yourself anyway, so pointless trying to learn something. 
you would be better off learning the methods of construction so you can employ trades as and when you need them. 
 

you will need a fair chunk of money for the extension, don’t for a minute think it will be cheap. 

Thanks Russell. I completely get it. Extension won’t be ‘cheap’ - not expecting them to be. I’ve previously hired builders to build extensions, full refurbishment etc. albeit not close to any of the actual work. But the recent quotes are eye watering, leaving no choice but to look for alternatives. But reading and YouTube can teach you only so much, if I have an option to spend that extra £5-6k to learn a thing or two ‘on the job’, at least I’d know what NOT to do during the extension build. It’s a basic brick box, no bells and whistles, so being fairly realistic in my expectations too.

[saveasteading]

The sun room could be fairly cheap.

Assuming all diy.

Concrete 20m3   or a lot of mixing.

 

Hardcore from demo company. 

mesh has to be bought

Timber £1,000 or look for reclaimed.

Windows second hand on Facebook or ask a window company ( they take old units out) . Felt, fixings etc 

Got a trailer or mate with truck?

Buy used mixer,   hire roller etc

 

Anyway, with lots of time and bargains, £5k target but prob more.

 

 

 

[kandgmitchell]

My error as well, I thought it was the extension that was 5m from the tree. If I was was building a non-bldg reg outbuilding that close to a tree I'd be looking to do it in the simplest manner possible, as unless you do some serious foundation work the tree will have an impact. 

 

So I'd go for a lightweight flexible structure on a simple base. Building a rigid masonry box that close to a big tree is asking for future issues