JackOrion

Been a while since I've visited the forum but interesting to see these figures. We're in West Yorkshire, and were quoted somewhere between £5500 and £7000 per sqm (!!!) earlier this year from main contractors. Single storey 120sqm building. Timber frame to almost passivhaus standard / triple glazed windows / MVHR etc.. Our plot also ticks all the £££ boxes: Tricky access, sloping site, piled foundations. Architects were designing to a budget of around £3k per sqm. Way off. One contractor actually told me their QS had priced it so high as they were looking at the final value of the house and bumped it up accordingly. (Is that standard?!) We're still having to go through some serious value engineering, and looking likely we'll have to do much of the project managing from weathertight shell onwards ourselves.

The proposals we have had so far include three different piling techniques - but all recommend rock sockets for the extra stability this will provide. For context, our plot is on a south facing slope within quite a steep sided valley, which might help explain the belt and braces approach.

Thanks. Yeah, the plot was about 30% market value due to a legal issue that was relatively easy to sort! The ground was at that point was of course unknown... but seems like it will be the factor that evens things out! Win some lose some. We'll see.

Many thanks for this @saveasteading – I'll get all the info together and get in touch with some more companies once back next week. I'll keep you posted on progress.

Thanks @ToughButterCup – yes, I'd read your previous posts with interest... I can't reach their website at the moment, but I see from their Facebook page that they sponsor a football team not too far from us. Will definitely be giving them a shout, though I fear access constraints will rule them out.

Thanks @saveasteading I don't have the ground report to hand at the moment (also, it's a huge file), but in short, bedrock is indicated to be 8m below ground. The 10m depth includes 2m drilled rock sockets. It's a relatively lightweight building (pile loads calculated at 75kN compression). Yes, the slope is stable. This has been established by the original ground investigation engineers plus at least two other geotech consulting engineers. That said, the rock socketed piles can withstand potential lateral force from this slope if required. The plan is for the slope to be more stable after construction. This is a good point. Initial thoughts were they wouldn't be interested in a small project and would charge the earth... but no harm in trying. Assuming you're referring to the likes of Bullivant / Van Elle etc? Any others from the top of your head?

Thanks @Russell griffiths I think that's where we're at. The quote we have so far is for a mini excavator with a drill mast (which will work to install the piles on the slope as well as on the flat part of the plot) The problem with the small diameter piles is as you say, many more are needed (£££) than if we had a more regular rig installing 300mm piles for example. Will follow your advice and reach out to more companies though. Agreed, it's definitely not looking like a cheap job.

Ah, I see what you mean there now, helpful to see it. I think the problem here comes down to how far beneath ground 'something solid' is...

Would be interested to see it!

@Russell griffiths @ProDave 3D model attached here, alongside elevation showing sloping 'stilt' detail. (completed design has quite a bit less glazing etc, but this should give the idea) The new house is in two parts and wraps around the hillside, utilising the flattest parts of the plot (and avoiding protected trees). Existing structures are greyed out – it's an old walled garden, which was pretty much a gardeners' tip when we bought it, but is in the grounds of a listed building (downhill, out of shot). So along with the geography we are fairly limited to what we can do. Interestingly, the existing brick potting sheds (greyed out on the left of 3d model) are on minimal foundations (built in 1800s), and slope downhill with the natural terrain, and haven't moved an inch. Image 1.pdf Image 2.pdf

Exactly that! 2-3m refusal on each ground investigation borehole would have been a much better result for us.

Funnily enough that's pretty much what we've got... the slope is only beneath one part of the house, which is already split into two sections to make the most of the flattest areas of the plot. At this point the building is essentially on stilts with a steel frame above the pile caps.

@Great_scot_selfbuild Thanks for this. Will check these out. Given our position, anything man-handleable sounds really appealing but again I suspect they won't do the trick for our ground conditions, as previous screw / helical piles were deemed unsuitable. I'll keep you posted on how we get on though. Likewise keen to hear how you get on...

A photo attached here.. although I suspect you were after a borehole log? The ground investigation took three boreholes... in summary they found up to a metre of 'cohesive made ground', below which is 'an extensive thickness of reworked clays with occasional gravel lenses revealed to variable depths' (4.5m to 10m), beyond which was the rockhead. strata.pdf

Unsure if we're going down the prefab route yet or not, but certainly can't get an artic anywhere near site. Spoke to a few suppliers (e.g. Eden Insulation), who suggested dropping their kit off at a delivery yard nearby, which would then be shuttled up to site on flat beds. The alternative would be to stick build. Pretty sure helical piles have been ruled out for us due to the ground conditions. Same goes for any major excavation of the slope – would likely require soil nails (more cost + large plant required!)