Piles for extension - what would I be looking at price wise?

Hi all,

Currently having a single story extension outside the back of my 1970s Wimpey-built home. (Every other house in my row has done the same). Note garden is level out to 7 metres away from the house then slopes downwards.

The extension is going to be 3m 40cm by 7m

what happened last week was that the builders had dug the foundations out to 1m deep in a "C" shape.

However building control came round and said that it was all "made up" ground e.g. they hadnt hit solid rock yet that there was also a privately owned sewer pipe running underneath the foundations (not a Wessex water owned one - I understand that can get expensive.

Builder is not keen on continually digging until they hit rock - they reckon it might be another 1 metres or more away. They would also then have to dig another 1 metres underneath the pipe and encase it in sand (I think). So builder not happy about getting in a trench that would be about 3 metres deep.

Building control have therefore recommended that we could drive piles into the ground. They said I need to supply them with.

1. A Drawing/Design for the piles.
2. A Drawing/Design for the ground beam.
3. Possibly at some point Structural calculations – to demonstrate design is structurally sound. (I think this is some kind of formula? - Hinley formula or Hellman formula or something like that!)

I've had two companies out so far- one said maybe about 12 piles needed - another said probably not that many. and am waiting for the quotes to come back.

What would you expect the quotes to be, if they are driving say 10 piles to a depth of 3m. I want them to also do the ground beam and pour concrete. Basically so the builder is ready to carry on.

Small diameter piles and 300x300 ground beams are likely to be cheaper than trenchfill to the centre of the earth.

Assuming it's 3m out from the existing building and 7m parallel to the rear wall (not that it makes that much difference if it's the other way around), six piles will suffice and roughly 16m of ground beam. Note that although the depth to good ground might be 3m or so, the piles are likely to be deeper, say 8m, as most of the loading will be taken by the base of the pile; there is some skin friction, but also negative skin friction from the fill material.

So, say mobilisation is £500, six piles x 8m long x £50/m is £2400, ground beams say £100/m is £1600 and then assuming that the floor slab is cast in with it at say £100/m2 that's another £2100, so all-up you're looking at in the order of £6600 plus VAT to get out of the ground to dpc level. you could use pot and beam floor instead of reinforced concrete, works out about £30/m2 placed ie £650.

This is on the assumption that driven steel-cased piles can be used, but there might be issues with vibration damage to neighbours (not if they're on piles themselves) and you might need Party Wall etc permissions as well, which could add another £1500 to the bill if the neighbours don't give you the ok without involving PW surveyor/s.

I don't know where you are, but there are two good piling contractors in this neck of the woods who do national work: use Yell.com to find 'em, one begins with D and is based in Attleborough, the other with V, in Dereham (this one has other offices all over the place). Not connected with either, just use them a lot on my jobs.

There won't be fixed quotes from anyone: they will assume a depth of pile, with credit and debit rates for length variations; debit rate will be a lot, credit rate will not be much. Review the quotes closely, the one that appears cheapest at first glance may not be so when you look at say an additional 2m of pile length.

Finally, there are smaller concerns about who would probably do it cheaper, but keep away if they aren't run by chartered civil or structural engineers. The old adage about peanuts and monkeys holds true...

Don't be tempted to use a raft foundation, btw. It will want to tip away from the building due to the load centroid being further out and relies on good ground preparation and consistent material across the building footprint. And it will surcharge the buried drain anyway, which is a no-no.

Wouldn't these piling companies need to work to an engineers design rather than some random piliing and beams?

Engineers fees will be extra £1k plus?

Also negotiate a reduction in the builders price if he is no longer doing the strip dfoundation he has priced for

^woody^ said:

Wouldn't these piling companies need to work to an engineers design rather than some random piliing and beams?

Engineers fees will be extra £1k plus?

Also negotiate a reduction in the builders price if he is no longer doing the strip dfoundation he has priced for

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That's why I said go to one with an SE/CE at the helm. Those figures should allow for SE design fees, if they outsource it; I do a lot of design for the ones beginning with D cough JE and it normally works out about £350 - £400 plus VAT for a small-scale little extension like that - calcs and a quick sketch is enough to satisfy LA and get it built. Appointing an engineer direct by Steffer will result in far more detail than is required and a bigger fee to him as a result.

Good point re builder claw-back, forgot to mention that.

Thanks for you advice - the builder did mention he had priced for £1000 worth of concrete - what kind of reduction in the bill should I ask for?

Basically all he has done so far is dig the 1m trench all the way round in the C - shape.

What else would he have to have done, that now the Piling company would take over?

Depends how much concrete he allowed for in his price. Readymixed works out between £65 to £80 depending on if full or part load for Gen1 mix, so if you say there would be about 9m3 in yours to 1m depth, in one pour that would be 6x65 full load + 3x65 + 80 or so for part load, comes to say £600 plus VAT. You'll obv have to pay for the excavation men and plant; and you'll need to backfill too, so more charge there. Hopefully the spoil is still on site...

The piling contractor will usually expect for the general contractor set out the pile positions, prep the ground and dig out for the ground beams after the piles are installed.

I've done quite a number of piled foundations around Bristol and South Glos last year - and this. I think you can expect quotes of around £4500 to £8000 including the beam. I generally find the companies will provide calcs within that as well. If you email me I can let you have a company that has been consistently cheaper, and have provided good service, over the last couple of jobs.

Hi Jeds,

I tried e-mailing you bto get the names of the companies but it wouldnt let me - says I need to add you as a friend or that you need to accept me?

I sent you a comment instead

Just an update on how it all went!

Piling cost £4300, ten piles, including all the concrete and reinforced beams etc.

Was amazed how quick it all went in, took four days to do, down to 3m.

Used a compressor on the driveway with air cables coming round the back of the house with a big torpedo shaped air hammer than made them fly into the ground! When I saw how easy it banged them in it made me realise how soft the made up ground must be!

Looks like the business to be in! £4300 for 4 days work (all the equipment is rented, and the two chaps doing it were only young) seems pretty good wedge for the piling company owner!

Had one other quote for £8000!!!

Managed to claw back £700 off the builder for the concrete that he no longer had to buy.

Never bothered with the structural engineer as the piling company outsourced that!

Anyway building continued and its now all finished and am enjoying the vastly bigger house!

THANKS A MILLION TO ALL THOSE WHO LEFT COMMENTS!!!

glad it went ok. Any pictures of the piling? how wide were the pipes and did they bang them in until they hit rock? or just go 3m then stop banging? Cheers

Hiya may i ask which piling company you use at the end? i know your project was nearly 10 years ago but I had the same problem at present. Many thanks

^woody^ said:

Also negotiate a reduction in the builders price if he is no longer doing the strip dfoundation he has priced for

Click to expand...

But not too much. Both piling prices we had recently expect the main contractor to all the over-site dig etc.