Extension foundation depth?

davidmcgee

Hi guys,

Have a question regarding building foundations.

We have just bought a house which has a conservatory (4m x 4m) which has a dwarf wall with 300mm wide walls. We spoke to the company that installed it and they said they would have gone down 600-1,000mm for foundations depending on soil conditions. They are a reputable company, I don't see any reason why they would lie about that - we'll need to dig some small exploratory holes anyways to check the figures exactly.

We would ideally like to strip it all down to the dwarf wall and continue building a single story extension up from there in conventional brick and standard roof (assuming the dwarf wall is OK, could always knock that down and start again, though).

My question is: From things I have read online it says for you need a minimum 1,000mm concrete foundation for a single story extension. However, this evening I was talking to a friend who runs a building company who says that 600mm depth and 500mm width (assuming 100mm either side of the dwarf wall) would be more than satisfactory to build upon - assuming the ground around the 600mm mark is OK - and that they as new builders only go down 600mm most of the time (more if needed).

Is this correct?

Obviously this is all hand-wavey at the moment, and obviously we would get building control in to assess, but:
1. Assuming the foundation depth is 600mm,
2. Assuming the soil beneath that is OK,
would that be possible to build upon?

Also, if traditional brick/roofing might be too heavy, would there be a more lightweight option in materials that we could go with that would allow us to use the same foundation?

Many thanks for reading this long essay and multitude of questions, but just at the start of this!

:beer:

EachPenny

Three questions:-

1) Are there any trees or hedges near the house?

2) What kind of subsoil is the conservatory built on?

3) Is there any evidence of previous excavation/alteration to the ground - e.g. ponds, ditches, retaining walls?

Trees and hedges cause issues with ground movement, which is more of an issue on soil types (e.g. clays) which shrink or swell as moisture levels change. Foundations on clay soils near trees need to be deep enough to avoid this problem.

Disturbed ground might not offer the same levels of support as undisturbed ground and can give rise to differential settlement and other problems.

A neighbour wanted to build a single storey garage consisting of 4" brick walls with a corrugated steel roof, about as light as you can get. His foundations ended up going over 6 feet down because the spot he picked for his garage a) was near a hedge, b) was on clay and c) clipped the corner of a filled-in pond.

So unless you know exactly what you are building on, it is essential to assume the worst when it comes to setting a budget for the foundations.

Furts

Without seeing and inspecting nobody can give a firm answer but basic principles are this. Conservatories are unregulated work, that is not subject to any design or inspection of the foundations. Basically anything goes. Consequently your foundations might be 600 deep, but equally they could have been cost cut to 450. But if the ground conditions are bad the crew digging them may have gone to 1000, but this will be unlikely. Time is money, and all this material incurs a cost with disposal, and then there is the additional costs for the concrete.

The other fundamental is this. There are huge cost incentives, and profits to be made, by keeping conservatory foundations scimpy.

The bottom line is it unlikely your foundations will be acceptable for future building, unless they received an inspection and sign off when they were dug.

davidmcgee

Thanks for answering, EachPenny.

1) Are there any trees or hedges near the house?

There are no hedges, but there is a single solitary tree about 2 meters away. It's trunk is pretty small and not very thick, maybe reaching 6 feet, and it has spindly branches reaching up to around 12 feet.

2) What kind of subsoil is the conservatory built on?

I'm not entirely sure yet as I haven't dug an exploratory hole yet.

3) Is there any evidence of previous excavation/alteration to the ground - e.g. ponds, ditches, retaining walls?

There shouldn't be any previous excavation/alteration to the ground - it's a new build (20 years) with a single previous owner who had the conservatory installed 11 years ago.

Davesnave

davidmcgee wrote: »

We spoke to the company that installed it and they said they would have gone down 600-1,000mm for foundations depending on soil conditions. They are a reputable company, I don't see any reason why they would lie about that - we'll need to dig some small exploratory holes anyways to check the figures exactly.

Talk is cheap. As Furts says, conservatory companies may say the right things, but it's what they do that matters.

Mine talked the talk, but cut corners attaching the superstructure. Once discovered, this took 4 man days to correct under my supervision, since their 'surveyor' didn't deign to turn up. He was complicit in their shoddy approach.

Bear in mind this is a company trading for 40 years with mainly 5 star Trustpilot reviews.

I can only guess at what they'd have done with the foundations, bearing in mind that there was no effective supervision, as there would be with regulated work. Thankfully, I did those.

You're guessing too. Get digging!

davidmcgee

Absolutely, Davesnave.

I am aware that conservatories are an unregulated mess (one of the reasons for knocking it down), and that salesmen will say anything!

However, this post was more about "Oh, I thought foundations would need to be a lot more substantial than 600mm for a single story extension, but that might not be the case?" And it turns out, I think, that 600mm could be fine in some conditions?

I'll get digging soon!

Furts

davidmcgee wrote: »

Thanks for answering,

There shouldn't be any previous excavation/alteration to the ground - it's a new build (20 years) with a single previous owner who had the conservatory installed 11 years ago.

Two points. First, it is what the ground was before the new build was constructed there. That is what you are going to determine.


Second, twenty years sets alarm bells ringing. You may be fine but an uncomfortable truth crops up. This is the early days of a significant move in UK mass house building. Brownfield sites, old industrial areas, contaminate land and all sorts was being used for the first time for house building. If your home is on such ground do not assume you have virgin ground before your home was built. In turn this means you may not have simple foundations.

Furts

davidmcgee wrote: »

Thanks for answering,

There are no hedges, but there is a single solitary tree about 2 meters away. It's trunk is pretty small and not very thick, maybe reaching 6 feet, and it has spindly branches reaching up to around 12 feet.

The pragmatic answer is this has to come out - you cannot have a tree 2 metres from your home, all the more so if you do not know the species. Forget your extension for a moment - how do you know what damage the roots might be doing to existing services and drainage?

Furts

davidmcgee wrote: »

Absolutely, Davesnave.

I am aware that conservatories are an unregulated mess (one of the reasons for knocking it down), and that salesmen will say anything!

However, this post was more about "Oh, I thought foundations would need to be a lot more substantial than 600mm for a single story extension, but that might not be the case?" And it turns out, I think, that 600mm could be fine in some conditions?

I'll get digging soon!

600 is not fine unless you are on rock! 900 would be the minimum, but folks always use 1 metre as the yardstick.

davidmcgee

Two points. First, it is what the ground was before the new build was constructed there. That is what you are going to determine.

Second, twenty years sets alarm bells ringing. You may be fine but an uncomfortable truth crops up. This is the early days of a significant move in UK mass house building. Brownfield sites, old industrial areas, contaminate land and all sorts was being used for the first time for house building. If your home is on such ground do not assume you have virgin ground before your home was built. In turn this means you may not have simple foundations.

It used to be farmland.

The pragmatic answer is this has to come out - you cannot have a tree 2 metres from your home, all the more so if you do not know the species. Forget your extension for a moment - how do you know what damage the roots might be doing to existing services and drainage?

That's a good point, and one that we'll need to explore. One problem is that it's on our neighbours property, but can always ask them/pay them to remove it. They might, of course, say no.

teneighty

It is pointless speculating, you will just have to dig the trial hole and see what type of sub-soil you have and how deep the existing foundation is. The 1000mm you refer to only applies to clay type sub-soils as they are affected by moisture and temperature, sandy and gravelly sub-soils and many others can be shallower provided you get down to a good solid undisturbed level. The ground can be surprisingly variable with adjacent plots having marked differences.

I once had a job many years ago where the homeowner was doing a DIY extension, he had that figure of 1000mm in his head so spent 2 weeks breaking out solid sandstone to get his foundation trench to the "correct" depth.

I did 3 extensions based on old conservatories in the last couple of years, 2 were OK and one needed underpinning. 1 was on sandy gravel, one was on chalk and third was very shallow just on the top soil hence that one needed underpinning.

PS. Likewise the trees are only an issue if you are on clay and then it depends on the type and size of tree.