Extension Foundations

11 Feb 2008
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United Kingdom
I have decided to build my single storey extension (24 sqm) myself but obvioulsy using trades where i need them like brick layers, plumbers, electricians.

I have had the drawings done and planning agreed by the council. They has specified 600mm wide by 1metre deep. The extension will be built with block and then rendered to match rest of house.

I will get a man with a digger to dig the foundation trench and then will get a concrete company to deliver the concrete. I have been advised the following and i just want to confirm:

1. The trench will be 1m deep from lowest lying point so could be more than a metre in some places?
2. I will need a polystyrene sheet in the foundation trench to barrier where it joins the house for expansion purposes.
3. The concrete should be poured to a level equivilent to 2 engineering blocks plus mortar joints below DPC so that the DPC can be laid at same height as exisitng house and then celcon block on top

Is all this correct? Also what do i use in the founcation trench to go over the soil and drainage pipes? so once the foundation has been poured the can be accessed?

Thanks in Advance

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points 1 and 2 seem ok.

regards point 3:

we allow two courses of block plus one of brick (internal skin). this allows you to easily remove the bricks where the screed needs to meet an external door threshold.
should you be laying an-all-in one power floated concrete slab, disregard the above.

regardless of the above statement, you should be showing engineering bricks externally where ground conditions dictate. if the ground falls away sharply then any exposed masonry below dpc should be visible as bricks and not blocks.

any drainage passing through the foundations can be either linteled over where it passes through the masonry or shuttered and linteled where it passes through the concrete.
Evening Nose,
That sounds about right,
I always used to recomend trench fill to 6" below finished path.
(don't send the brickys down the trench daddy)
As Ian will be using tradesmen to do the build , they should be able to advise him on wether or not the founds need to be stepped etc.
Thanks guys....

So at the edge of the house ground level is approx 6in below DPC and 10ft away it is about 12in below. The architect has specified 1m x 600mm foundations and celcon block construction as the extension will be rendered with pebbledash to match rest of house so do i need to worry about whether the engineering blocks are visible.

So below the celcons do i have to use engineering block or brick, what is best and how many layers before the celcons would be best practice? What i'm trying to get to is work out the depth of concrete i need to order for the trenchfill? Also what is the easist way to mark the foundation depth around the trench beofre pouring?, someone has said use wooden pegs other has said use steel rods?

Thanks again

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Hi, Ian,
don't worry about the fall off in the ground level, you can decide to keep the path/ ground level at the same as the house with a bit of infill at the end of the job when the paths are being laid, so regard it all as level.
If you can get a readymix waggon up to the foundation trench then it is cost effective to trench fill to 9" below finished path.
If not and it's a mixer job, then keep it to 12" or so depth.
Using wooden pegs is the usual way to level the concrete, I'm sure it's been mentioned before elsewhere, but any DIY manual will tell you how to go on.
You had best ask the BI what he wants to see below DPC some allow flettons some like engineering brick, some allow trench blocks.
But the last three course should be in engineering brick and face pointed.
Thanks Chessspy

When you say finished path do you mean normal ground level which i was told is minimum of 15cm below DPC?

Also how would you lintel over the soil pipe in the foundation trench do you buy a pre made up lintel or make it yourself using block?

Thanks again Ian
Hi, Ian,
finished path can be deemed to be no higher than 150mm below DPC.
It can be lower but not higher.
You can alow it to go lower by letting the path slope, but make sure you've got enough courses of brick faced up below ground just in case they show.
You wouldn't normally slope off the path more than one course extra, so you would make up the ground around the extension where there is going to be hard standing (ie. path) with type 1 (chalk or small hardcore) and sharp sand on top of that to lay the path on (if slabs).
the garden can be allowed to fall away in it's natural slope if necessary.
to bridge over a pipe in the trench, it is normal to box round with temporary wood shuttering, it doesn't need to be fancy just alow some room around the pipe for pea gravel and make sure you will be able to get the shuttering out after you have poured concrete.
Then a pre stressed concrete lintol (or two) to take the wall(s)
length is more or less immaterial as long as the land is 6" or more.
Keep the dig clean and tidy and the DS shouldn't give you grief, but will give good advice regarding the bridging. (of the pipe)

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