Foundations: cost relative to extension

Dedekind

Hi, this is me again with another question regarding a single storey rear extension. All the builders quotes we have are based on 1m foundations for the new extension. There is always the possibility that a building inspector requires deeper foundations (there is for example a large tree about 8m away from where the foundations would be).

Let's say they decide we need 2m foundations instead of 1m -- how much is this going to affect the cost? The extension is roughly 3m x 3m. We don't have a quote for just the building work, since the quote includes demolishing a current extension, fitting some windows, electrics, plumbing, etc..

HHarry

Some of the costs are proportionate - twice as deep, then twice as much soil to remove, and twice as much concrete to buy (assuming you’re just going to mass fill).
Below a certain depth (1.2m ish) you’ll need to shore up the sides, and it can get tricky with digger reaches, and needing to get people in the trench.

It’ll be tricky to get an idea of the extra cost.

Tucker

When I had my extension done it was either standard strip foundations a metre deep or it would require mini piling. 
The cost of that was over 3 times as much. 
You might not get a definitive answer until you dig out but I rang them and they said the area wasn't always suitable for standard foundations. They suggested I could do a trial dig where I dug out a patch of 1m square and 1 metre deep and the came and checked it and gave a strong indication that it was fine for standard foundations. 
Perhaps speak to Building Control. They will know what foundations have been used nearby in your close location and will probably give you an idea of what is likely where foundations are concerned. 

bozzy18

What about ground screws instead of digging deep - is that feasible?

FaceHead

Have you had a structural engineer specify the required foundations? And then sent them over to building control for checking and approval. The builder then quotes for, and builds, what the engineer has designed and building controller has approved. 

A 1m foundation should be adequate for a single storey extension in most usual situations, but the engineer and building controller should be aware of the tree. 

Foundations get ever harder to dig as they get deeper, with sides collapsing and what not. With very little to go on, your 6 linear meatres of 1m deep foundation might be a couple of 000's, and doubling the depth might add twice that again. If for some reason you think the tree might require you to go to a 2m foundation, perhaps you need to brace yourself for a 4k extra. Better yet, discuss the eventuality with your builder before work begins. 

Out of interest what is the total cost you're looking at for this 9msq extension? 

stuart45

It depends on the type of tree and whether the sub soil is high, medium, low shrinkable clay or something else like chalk.
Building Control use this as a guideline NHBC Standards 2011 (nhbccampaigns.co.uk)  but until the foundations are dug out you never know for sure if they will pass them. Sometimes you come across a problem with made up ground(although this is quite rare).

Dedekind

Thanks. The structural engineer is finalising the drawings these days, and the builder knows about the tree and is of the idea that it won't pose any problems but cannot guarantee obviously.

If what we need to add is 4-5k, then I would very very much would try to avoid it as much as possible; however it's not exorbitant. I was afraid that an extra 10k or so would be needed..