Foundations for small timber extension in restricted space

27 Feb 2018
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United Kingdom

I'm planning a small (2m x 2.5m) timber-framed utility room extension on the front of the garage / side of the house. The driveway is only just over 2.5 metres wide, with our house and the neighbour's converted garage either side. So I'm wondering about the likely best / most cost effective way to do the foundations.

I need to take ground level down by about 400mm anyway across the area to accommodate hardcore/insulation/slab and match up to the floor level of the garage.

I'm not certain exactly how to identify the soil type, but I've lifted a couple of paving bricks and dug down 500mm or so below the sand / hardcore the soil and it still seems fairly soft. The other complication is that we're on a hill - the neighbour's garage floor is about 1m below our ground. So if I have to take foundations below the bottom of their building that could end up pretty deep.

But I think with such a narrow space it's going to be pretty awkward and slow to dig trenches and remove spoil with a digger - certainly for the front wall across the drive.

Any suggestions? Is it likely to be worth exploring pads / piles / rafts (and paying an engineer to design them) cost-wise? Or should I just suck it up and get the spade out?

Also, I'd probably want to use readymix concrete. If I did go for trenches, is it possible to pour them and the slab at the same time? I think would be the most cost-effective delivery for a job this size?

Just hoping to specify something that's got a reasonable chance of building control being happy with it and a limited risk of having to change plans / regret the approach once we start digging...
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