Foundations conundrum

2 Dec 2010
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United Kingdom
Advice on digging extension foundations needed, please! Sorry for the long post but I wanted to try to be clear.

I'm building a 2-story extension to the side of my house, as shown in the plans below. The extension is marked "dining room". Note how the end of the extension near the living room joins on to the chimney.

Now, I've stared to dig the footings, which the BCO stated he wanted to be the same depth as the house. Fair enough. At the end of the extension that joins the house next to the kitchen, the foundations are 1800mm deep (lots of digging!). However the house is built on a gradient, so I anticipated that the end nearer to the chimney would be not so deep.

On uncovering the existing footings at the chimney I have hit a problem, as shown in the photo.

The chimney foundations are higher than the house foundations. Unfortunately the chimney footings are not wide enough to support the new wall with a 150mm excess, so i have dug round them so that they can be widened. However this has uncovered that the house foundations at this end are a) higher than the other end of the extension (so there must be a step somewhere) and b) go deeper than the chimney footings.

So, what to do?!

Options as I see it are:

a) have 500mm deep footings at the chimney end, with lots of steps dug round to the other end where the concrete will be very deep or have lots of trench fill blockwork.

b) dig down next to chimney footings to depth of house footings, but do not undermine chimney footings. Concrete at whatever depth, e.g. 900mm to match existing with possible steps.

c) undermine chimney foundations (!!) and concrete underneath them maybe with some reinforcing mesh.

d) fill the footings back in and forget the idea of an extension!

Any advice appreciated!

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Thanks, it's another option! Though the work is being done under a building notice (rather than full plans) so not sure how enthusiastic he will be about providing advice (rather than telling me to hire a structural engineer).....?
What ever you do, don't abandon the extension project.

Lets have a thread on DIYnot where someone is actually doing some DIY for a change ;)

PS milk, no sugar, large mug please.
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Go to the depth of the house fitting and underpin the exposed chimney footing.
Wavetrain, option D is out anyway - I have filled three 6-yard skips so far so I don't have anything to fill the hole with! :D

Jeds, thanks for the reply, could you explain what you mean by underpinning? Do you mean installing a pile and a cantiliver beam of some kind under the chimney foundation ?? Sorry if this is a stupid question by the way, I just associate underpinning with lots of expense (and there is no access to get a pile driver in position anyway). Alternatively are you referring to my option C, which involves excavating under part of the chimney foundation?

I'd be inclined to fill the trench with concrete where you've undermined the chimney foundation. How about stopping your extension foundation against the chimney foundation and bridging between that and the existing house wall with a concrete lintel (140x100 should easily span a metre) to carry the inner leaf past the side of the chimney? The outer brick leaf could attach to the chimney with a wall starter.
Hmm, a lintel over the chimney foundation is an interesting idea. However I do foresee some installation issues, since being below DPC I imagine that the cavity of the existing wall has been filled with concrete. Would you be proposing to support it only on the 100mm external leaf?

BTW yes the outer leaf is planned to attach to the chimney with a wall starter. Although with this method it will be supported only by the chimney foundations for the first 300mm or so, before dropping on to the new footings.
I really meant just exacavate the foundation trench and float concrete under any of the exposed chimney foundation. If necessary you could leave the bit under the chimney 70 or 80mm short and dry pack it once the concrete has set.
I would suggest underpinning your chimney but as another member has already mentioned, bco will tell you how they want it. If you listen to bco they are there to help.

How have you managed to build a two story extension without planning permission? 4m is the max for a roof without full plans, anything taller than 4m and you need full plans? If your trying avoid getting full plans, I hope you have good neighbours, if 1 complains ooohhh ddeeaarr!!!
How have you managed to build a two story extension without planning permission? 4m is the max for a roof without full plans, anything taller than 4m and you need full plans? If your trying avoid getting full plans, I hope you have good neighbours, if 1 complains ooohhh ddeeaarr!!!

OP hasn't said that he didn't have or need full Planning Permission, in fact he hasn't mentioned it at all.

He's only talked about Building Regs.
He's referring to Building Regs, not Planning Permission. Plans for PP do not require the same structural detail that BR do.
Just in case anyone looks at this thread in future, I thought I'd let you know that I have agreed with the BCO to lintel over the chimney foundation, pocketed 100mm into the house wall. To be honest, based on ground conditions I think its a bit of a waste of time since I had to dig through 2ft of good clay to get level with the bottom of the house footings; I think the chimney was added later when they realised they didn't need to go down so deep!

I'll try to remember to take a photo tomorrow once the lintel is in.

By the way, I do indeed have planning permission! Full plans building control is three times the price, and seems excessive for a comparatively simple extension.


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