Extension foundation alternative

pedgepuk

I'm nearing a decision on a shortlist of builders to do a single-storey side and rear extension for a shell only where I would do most of the internal works. Some of the quotes have been within the budget and so I've brought the shortlist down to two reputable building firms who I've vetted and received good feedback on. The extension will be approx 6.6 x 4.9 metres.

One of the builders has suggested that one way to bring the cost down is rather than excavating in full for the 1 metre trench foundation to be filled with concrete and made up of block, that they would excavate at certain points where the 1 metre depth would be required in order to take any loads, which what he called are pad foundation with beams. The saving here is on labour, excavation, skips and concrete.

I live on partial clay and chalk, and my concern is on any potential ground movement due to clay, and if the beams from one pad foundation to another would be in some way be affected in future.

Has anyone had anything similar been done or have advice on this approach?

TIA

grumbler

AFAIK, at the end of the day it's the building inspector who makes the decision about the foundations, at least about the depth.

Ultimately, the purpose of any foundation is to spread the load. I am no expert, but I expect the total area of padstones to be about the same as of a strip foundation, and hence the volume of the soil and concrete.

DevilDamo

What does your structural engineer say?

weeg

What your builder is suggesting doesn't make sense. There's a lot more engineering in what they are proposing, and that usually adds costs. I'm not saying it couldn't work, but there is a good reason that that's not the norm - after all, if it was cheaper that's how every new build would be built. Strip footings are cheap. I'd also wonder why they are going down a full 1m, although obviously that depends on your soil and nearby trees.

FreeBear

weeg said: What your builder is suggesting doesn't make sense. There's a lot more engineering in what they are proposing, and that usually adds costs.

I'm also trying to see the logic as well... If the builder has a mini-digger on site, it isn't going to make any difference in time to dig the whole lot out to 1m. Assuming it is blockwork below ground, yes, it will use a few more blocks and brickie time, but this would be offset by a quick & simple strip foundation.

There would be a saving to be had on skip hire, but for £200 or so, is it really worth it...

Mickey666

Hmm.  Listen to the builder by all means but verify acceptability with the architect/structural engineer and building control.  If they don't agree then you won't get the work signed off.

I also agree with FreeBear.  How much is this 'novel' approach going to save?  A mini-digger can move a lot of soil very quickly and pouring concrete into a trench is faster than building blockwork.  I'd be interested to know what is the cost saving the builder is claiming with his proposed approach.

Doozergirl

I was thinking the exact same thing.  It's easier to dig one strip foundation with a mini digger than hand finishing numerous larger holes. 

Plus you need a structural engineer for it, which is an additional cost to the building inspector peering into the strip footings and giving a yay or nay.  

I don't believe it is cheaper at all.  As someone has said, we'd all be doing it if it were the cheapest way... 

pedgepuk

Thanks for the feedback all.

I've spoken to my structural engineer and also a building control surveyor, and both have said that it's a common alternative to a full excavation and trench concrete foundation. This is mainly used when there are drains or other obstacles, but it's perfectly safe as long as the pads take the load of the upper floor. I live on partial clay and chalk where the foundation depth has been 1000mm for similar projects or new build 2 storey houses.

I had my concerns as I never heard of this approach, but having now looked at online designs and also spoken to the SE and BC, they've provided confidence. Of course the BC surveyor may during the initial visit decline the approach once the trenches are excavated.

Also, having an extremely narrow road, skip access is tricky and also limited space to have larger skips, and more excavation requires more skips.

There seems to be a £2-2.5k saving on the initial costs but this is the south east.