Extension foundation depth?

davidmcgee

Hi teneighy,

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.

Absolutely, that's what I am going to do.

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.

So, just to confirm: You have completed three conservatories to extensions, and only one of them needed any foundation work?

teneighty

Yes but it is purely pot luck, I could do another 3 and they might all need underpinning. There is no hard and fast rule.

Davesnave

Furts wrote: »

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

The last one I built for an extension was taken to about 600mm then the inspector looked at the clay content and said "1 metre please!" In other words, the final decision was on-site.

That was in 1987. Requirements have probably become more stringent since then.

My BiL hit bed rock at around 500mm a few years ago, and that had to do, however. Short of blasting.....

OP, at this time of year, you may find the hole fills with water faster than you can dig. That was certainly true for me when our council insisted on 3 trial holes to expose my bungalow's foundations.

Another tip. If you have the space, consider garden re-landscaping if you need to dispose of foundations. Skips are very expensive.

LandyAndy

Furts wrote: »

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

Well, it made me smile.

Doozergirl

I have never seen a footing specified that was less than a metre before breaking ground.

I’m slightly baffled by your friend’s comment. It would take some persuasion to convice our BCO that less than a metre is acceptable, and I’m not sure I’d be brave enough to try! You don’t pour in a full metre of concrete though - 700mm is ordinary.

Foundations are about firm ground, not depth. You are at the mercy of the ground. We have had bore holes made for our new house and the foundation depth have been assessed off the assessment ofnthe ground beneath.

Our structural engineer says that a significant proportion of his work comes from conservatories with inadequate foundations. It is not the weight of the building, it is the state of the ground.

DaftyDuck

The way I would look at it would be: cost of savings... relatively small. Cost of redoing from scratch, with suitable depth and base, relatively small. Cost of it going wrong in two/five years for that saving.... potentially more than the initial build cost...


Saving therefore virtually pointless.

Itmay be possible and work. It's probably worth starting from scratch.

davidmcgee

It may be possible and work. It's probably worth starting from scratch.

I am a little confused. When building control/structural engineer comes round, and *if* they say the foundations are adequate for our works (might be a big *if*, but an *if* nonetheless), then what would the issue be?

LandyAndy

davidmcgee wrote: »

I am a little confused. When building control/structural engineer comes round, and *if* they say the foundations are adequate for our works (might be a big *if*, but an *if* nonetheless), then what would the issue be?

If the BC inspector says they are adequate then there will be no issue.


It isn't a 'big *if* though. It's a much bigger *if* than that.

kev25v6

For my single storey extension with a few small conifers near by the building inspector wanted 2m deep or bedrock, which ever came first, luckily they hit bedrock around a metre down.

Ganga

My next door neighbour had a single storey extension constructed on the back of her house to extend her kitchen,they did not dig down very deep but the builder told me it was a raft and i watched them put steel reinforcing in the foundations before pouring the concrete.