JackOrion

Thanks for this. I think in our area we need to allow for a 1 in 100 year storm event, so yes, 3m³ would likely need to be increased. Inspired by some other threads on here and reading online, we're now looking at multiple SuDS planters, a substantial amount of rainwater harvesting, and splitting into smaller soakaways from that point. Hopefully we can find an engineer on the same page.

One of our planning conditions is to provide details of surface water drainage. The LPA and water company will only allow foul water into the sewer, so we need engineering calcs and drawings showing the soakaway design and location. Site details: Mainly sloping site Soil is very gravelly clay Test pits (400 × 400 × 400mm) drained quickly (in under 7 minutes) Roof area approx. 120m² Paths/driveways will all be permeable A few old Victorian garden buildings are on site with lots of old land drains dotted around (it's an old walled garden). No drainage issues anywhere and we've owned the plot a good few years. From what I understand this probably means we need around 3m³ soakaway capacity. We’ve had two drainage engineers visit and both suggested a single soakaway in a flat area (about 5m²) in front of the house. However, that spot isn’t 5m from the house foundations nor is it 5m from a retaining wall further downslope (neighbour’s property), so it seems like it will not meet the usual guidance, and could prove problematic in future. On the other side of the house we have about ½ acre of sloping woodland, with plenty of space but more difficult digging (tree roots present, and will need to be by hand), and then we have a more gently sloping footpath below that. I’m wondering whether splitting the drainage into several smaller / shallower soakaways in the wooded / footpath area might make more sense, which would keep everything well clear of structures. Does that sound like a sensible approach? Has anyone used any drainage engineers who may be a little more inventive than those who have suggested a single large soakaway, or perhaps have more progressive SUDS experience? Any recommendations / thoughts welcomed.

Invert is around 600mm below ground level. Excavation starts this week, so we will see how things stand once our groundworks contractor gets to grip with the levels. Thanks all.

We have a 100mm lateral drain that we will be connecting to. Runs right through our plot and is five metres or so from the edge of the house. This existing drain runs down a steep gradient (perhaps around 30 degrees), before it drops down to the mains below. We're in a valley hence the slope, and it's an old Victorian drain. We have been given the green light to connect by the water company for domestic foul water (with a soakaway for surface water). Who governs the connection method and gradient of our connection? Building control or utility company? Does anyone have any experience of connecting to a steep sloping lateral drain such as this, and is it likely to be a complex procedure? Have been reading quite a bit about maximum recommended gradients of 100mm drains being more like 14 degrees to avoid solids being left behind, and wondering whether the existing drain gradient makes our job trickier. We can control the gradient of our connection as we're on a plateau midway down the slope.

Thanks for this. Initially was thinking / hoping this could be a temporary solution as is.. but overlooked the need for second pump and volumiser. Building the house 2026, starting groundworks in Feb. It's about 25 metres or so across a largely paved old walled garden from this outbuilding to the house (2 bedroom single storey, near passivhaus timber frame). Maybe cheap ASHP is the answer for the outbuidings as you suggest.

Just had UFH installed in outbuilding (will be studio / workshop eventually / and temp accomm while building), using Willis heater. Two rooms approx 20sqm total. Seem to have the same issue as @oranjeboom did – Willis is 'kettling' when switched on, which is how I've found this thread. Before I get the plumber back.. could someone more knowledgeable than I check our arrangement and advise? Assuming we need a second pump for the heaters as first mentioned by @Nickfromwales ? Any help / thoughts appreciated.

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!