2 Storey Extension - Foundations

Hi there - completely new to all of this and feel like im going round in circles not getting many answers from building control or the architect. Please help!

Im based in Berkshire (seems like the costs in this area are very expensive!)

I have had plans drawn up and approved to put a 2 storey extension on the back of my house. 3 x 6 m2

Building control however infomed me that due to the soil type in the area I needed a geotechnical report carried out.

£1600 later the geotechnical report was carried out and a design was provided for a traditional trench fill foundation, taking into account the local trees, and a depth of approx. 2m was proposed with the provision of heave precautions to the inside face. This added an additonal 5k to the cost of the project. Making it around 12k in total.

I asked building control to provide me with alternative options as this additional cost would severely eat into our budget.

The other 2 options provided were

1. It may become more economically viable to consider the possible use of a pile and ringbeam foundation as less concrete is used and less waste is created that would need to be removed from site.

2. Instruct a chartered engineer to provide an engineer designed foundation taking into account the geotechnical report previously undertaken, a possible pad and beam design might be viable, again this would save on labour, amount of concrete and less waste to be removed from site which might also make it more economically viable.

I have contacted a company that specialises in Piling foundations and the quote that i have received so far came in at around 10k

The costs so far, just to put up the frame of the extension, is coming in at around 80k and thats before we look at fitting in a new kitchen, flooring, tiling etc.

If anyone can provide me with some advice on these options that would be great! Also do the figures sound about right for this area? Looking at the research it should cost £1,500-£2,000 per m2 which would be around 36k or am i missing something here?

 

your floor area is 36 square metres -you have to count every floor.

I would say £80k is in the right ball park for a 2 story jobbie in the home counties.

Im sorry the foundations are eating into your budget -its a common problem. Anything below ground is so unknown.

Digging down to 2 metres, foundation and the cost of an extra muck away is only about £2.5k I can only guess the builder is allowing for shoring works for the trench

extra concrete is about 8 cube which is say £1,000
extra cost of a grab lorry is £300
extra cost of claymaster sheets is £300
extra cost of labour is say £800

its not adding up to £5k -hopefully a pro builder will comment on this, he may well disagree
however please dont think Im saying your builder is overcharging -Im not seeing the whole picture and foundations are very difficult to price.

My only thought was perhaps you should see if there is a local groundworking company that specialises in foundation works that can price for just that part -they are companies that have their own equipment and thats all they do: drainage and foundations, sometimes upto to DPC sometimes just the founds.

 

Was it screw piling? Would screw piles and a ring beam be ok in this situation?

 

@Tigercubrider This is the response from a piling company that provided me with a quote. The other 3 complanies i contacted didnt come back to me so i dont have anything to compare it with at the moment or if there are any other options.

"We have allowed to install 300mm nominal diameter CFA bored pile, allowing for a Factor of Safety 3 which in
most cases will negate the need for any load testing.

Due to the granular unstable and water bearing nature of the soil’s, it is evident that standard open hole auger
bored piles would not successfully be installed. It is therefore proposed to install CFA piles."

@Notch7 I was worried that would be the case. The breakdown i have is below

Reduce Dig (dig out floors in new extension 420mm) £710
Excavation for mass footing Foundations £735
Muck away - Clean £900
Mas Concrete Foundations 10000x600mm £2,850
Underground Drainage (new soil pipes, manholes, clay pipes, gullys, soakaways £2,900
Skips/Plant/Mobile wc £4,000

I can challenge back on the skip hire as I have read that using a grab lorry is cheaper and the cost against the undergraoud drainage is an unknown. He has put it in there just in case as we have 3 manhole covers nearby. Great idea about talking to a groundwork company as this may save on some of the costs of hiring the plant equip!

Thank you both for responding. I'm certainly getting the impression that this is not plain sailing. It taken 6 months to get this far and we havent even started yet